This adds three new options to control the behavior of TCP keepalives:

- CURLOPT_TCP_KEEPALIVE: enable/disable probes
- CURLOPT_TCP_KEEPIDLE: idle time before sending first probe
- CURLOPT_TCP_KEEPINTVL: delay between successive probes

While not all operating systems support the TCP_KEEPIDLE and
TCP_KEEPINTVL knobs, the library will still allow these options to be
set by clients, silently ignoring the values.

When connecting to a domain with multiple IP addresses, allow different,
decreasing connection timeout values. This should guarantee some
connections attempts with sufficiently long timeouts, while still
providing fallback.

First off the timeout for accepting a server connect back must of course
respect a global timeout. Then the timeleft function is only used by ftp
code so it was moved to ftp.c and made static.

1- Two new error codes are introduced.

CURLE_FTP_ACCEPT_FAILED to be set whenever ACCEPTing fails because of
FTP server connected.

CURLE_FTP_ACCEPT_TIMEOUT to be set whenever ACCEPTing timeouts.

Neither of these errors are considered fatal and control connection
remains OK because it could just be a firewall blocking server to
connect to the client.

2- One new setopt option was introduced.

CURLOPT_ACCEPTTIMEOUT_MS

It sets the maximum amount of time FTP client is going to wait for a
server to connect. Internal default accept timeout is 60 seconds.

Do not try to resolve interfaces names via DNS by recognizing interface
names in a few ways.  If the interface option argument has a prefix of
"if!" then treat the argument as only an interface.  Similarly, if the
interface argument is the name of an interface (even if it does not have
an IP address assigned), treat it as an interface name.  Finally, if the
interface argument is prefixed by "host!" treat it as a hostname that
must be resolved by /etc/hosts or DNS.

These changes allow a client using the multi interfaces to avoid
blocking on name resolution if the interface loses its IP address or
disappears.

As a follow-up from commit d5b5f64b, clear the sock_accepted status
when such a socket is closed to avoid a re-used connection to retain the
state wrongly.

Bug: http://curl.haxx.se/mail/lib-2011-12/0079.html

Keep track of which sockets that are the result of accept() calls and
refuse to call the closesocket callback for those sockets. Test case 596
now verifies that the open socket callback is called the same number of
times as the closed socket callback for active FTP connections.

Bug: http://curl.haxx.se/mail/lib-2011-12/0018.html
Reported by: Gokhan Sengun

When the new socket is created for an active connection, it is now done
using the open socket callback.

Test case 596 was modified to run fine, although it hides the fact that
the close callback is still called too many times, as it also gets
called for closing sockets that were created with accept().

Moved out into a separate function to work as a "generic" socket()
replacement.

Previously the bit was set before the connection was found working so if
it would first fail to an ipv6 address and then connect fine to a IPv4
address the variable would still be TRUE.

Reported by: Thomas L. Shinnick
Bug: http://curl.haxx.se/bug/view.cgi?id=3421912

Save the errno value immediately after a connect() failure so that it
won't get reset to something else before we read it.

Bug: http://curl.haxx.se/mail/lib-2011-10/0066.html
Reported by: Frank Van Uffelen and Fabian Hiernaux

Renamed the variable from 'proto' to 'level' simply because it is not
protocol you set but level and that is the name of the argument used in
man pages and the POSIX documentation of the setsockopt function.

When connecting, make sure TIMER_CONNECT is set.

When using the multi interface, a SOCKS proxy, and a connection that
wouldn't immediately consider itself connected (which my Linux tests do
by default), libcurl would be tricked into doing _two_ connects to the
SOCKS proxy when it setup the data connection and then of course the
second attempt would fail miserably and cause error.

This problem is a regression that was introduced by commit
4a42e5cd that was introduced in the 7.21.7 release.

Bug: http://curl.haxx.se/mail/lib-2011-08/0199.html
Reported by: Fabian Keil

With "@unittest: [num]" in the header comment for each tested function.
Shows we have a log way to go still...

It was mostly typecasted to int all over the code so switching to long
instead all over should be a net gain.

Fix compiler warning: conversion may lose significant bits

Fix the return type of the callback to match close() and make use of it.

Introduced the initial setup to allow closesocket callbacks by making
sure sclose() is only ever called from one place in the libcurl source
and still run all test cases fine.

When connecting to a socks or similar proxy we do the proxy handshake at
once when we know the TCP connect is completed and we only consider the
"connection" complete after the proxy handshake. This fixes test 564
which is now no longer considered disabled.

Reported by: Dmitri Shubin
Bug: http://curl.haxx.se/mail/lib-2011-04/0127.html

"if(a)" is our style, not "if( a )"

By the use of a the new lib/checksrc.pl script that checks that our
basic source style rules are followed.

asyn-ares.c and asyn-thread.c are two separate backends that implement
the same (internal) async resolver API for libcurl to use. Backend is
specified at build time.

The internal resolver API is defined in asyn.h for asynch resolvers.

Found with codespell.

When checking if an existing RTSP connection is alive or not, the
checkconnection function might be called with a SessionHandle pointer
being NULL and then referenced causing a crash. This happened only using
the multi interface.

Reported by: Tinus van den Berg

Bug: http://curl.haxx.se/bug/view.cgi?id=3280739

Introducing a few CURL_SOCKOPT* defines for conveniance. The new
CURL_SOCKOPT_ALREADY_CONNECTED signals to libcurl that the socket is to
be treated as already connected and thus it will skip the connect()
call.

When the callback returns an error, this function must make sure to return
CURLE_ABORTED_BY_CALLBACK properly and not CURLE_OK as before to allow the
callback to properly abort the operation.

The idea that the protocol and socktype is part of name resolving in the
libc functions is nuts. We keep the name resolver functions assume
TCP/STREAM and we make sure that when we want to connect to a UDP
service we use the correct UDP/DGRAM set instead. This bug was because
the ->protocol field was not always set correctly.

This bug was only affecting ipv6-disabled non-cares non-threaded builds.

Bug: http://curl.haxx.se/bug/view.cgi?id=3154436
Reported by: "dperham"

As the function doesn't really use the connectdata struct but only the
SessionHanadle struct I modified what argument it wants.

When using the multi interface and connecting to a host name that
resolves to multiple IP addresses, there was no logic that made it
continue to the next IP if connecting to the first address times
out. This is now corrected.