Remove all system file includes from this file as they're not needed

Reported by: Dan Fandrich

When cross-compiling, CURL_CHECK_PKGCONFIG was checking for the cross
pkg-config using ${host}-pkg-config.

The gold standard for doing this correctly is pkg-config's own macro,
PKG_PROG_PKG_CONFIG.  However, on the assumption that you have a good
reason not to use that directly (reduced dependencies for maintainer
builds?), the behaviour of cURL's version should at least match.
PKG_PROG_PKG_CONFIG uses AC_PATH_TOOL, which ultimately ends up trying
${host_alias}-pkg-config; this is not quite the same as what cURL does,
and may differ because ${host} has been run through config.sub.  For
instance, when cross-building to the armhf architecture on Ubuntu,
${host_alias} is arm-linux-gnueabihf while ${host} is
arm-unknown-linux-gnueabihf.  This may also have been the cause of the
problem reported at http://curl.haxx.se/mail/lib-2012-04/0224.html.

AC_PATH_TOOL is significantly simpler than cURL's current code, and
dates back to well before the current minimum of Autoconf 2.57, so let's
use it instead.

A bundle is a list of all persistent connections to the same host.
The connection cache consists of a hash of bundles, with the
hostname as the key.
The benefits may not be obvious, but they are two:

1) Faster search for connections to reuse, since the hash
   lookup only finds connections to the host in question.
2) It lays out the groundworks for an upcoming patch,
   which will introduce multiple HTTP pipelines.

This patch also removes the awkward list of "closure handles",
which were needed to send QUIT commands to the FTP server
when closing a connection.
Now we allocate a separate closure handle and use that
one to close all connections.

This has been tested in a live system for a few weeks, and of
course passes the test suite.

On FreeBSD this fixes the warning:
Use of uninitialized value $p in string eq at /usr/local/lib/perl5/5.14.2/BSDPAN/BSDPAN.pm line 36.

small compilation fix

BLANK_AT_MAKETIME may be used in our Makefile.am files to blank
LIBS variable used in generated makefile at makefile processing
time. Doing this functionally prevents LIBS from being used for
all link targets in given makefile.

This handling already works with the easy-interface code. When a request
is sent on a re-used connection that gets closed by the server at the
same time as the request is sent, the situation may occur so that we can
send the request and we discover the broken connection as a RECV_ERROR
in the PERFORM state and then the request needs to be retried on a fresh
connection. Test 64 broke with 'multi-always-internally'.

Currently, LIBS is already used through other macros.

Although it is not explicitly stated in the documentation, NSS uses
*pRetCert and *pRetKey even if the client authentication hook returns
a failure.  Namely, if we destroy *pRetCert without clearing *pRetCert
afterwards, NSS destroys the certificate once again, which causes a
double free.

Reported by: Bob Relyea

Affected autobuilds: IRIX, AIX, Tru64 and AIX.

.. that are sent when auth-negotiating before a chunked
upload or when setting the 'Transfer-Encoding: chunked'
header and intentionally sending no content.

Adjust test565 and test1333 accordingly.

You can do custom commands to FTP without sending anything by using the
CURLOPT_NOBODY, which -I sets.

Blocking connect on the socket has been removed from opensocket
callback. opensocket just opens a new socket and gives it back to
libcurl and libcurl will take care of the connect. sockopt_callback has
also been removed, as it is no longer required.

AIX sys/poll.h header file defines 'events' and 'revents' as C
preprocessor macros. Usage of these literals in libcurl's external
API was introduced in commit de24d7bd causing AIX build failures.
Appropriate inclusion of sys/poll.h by libcurl's external interface
fixes AIX build and usage issues while avoiding a SONAME bump.

runtests.pl -am now uses the "PASS/FAIL: [desc]" output for each
executed test. You can run 'make test-am' in the root build directory to
invoke that. The reason for this output style is to better allow generic
test suite parsers to also grok our test output.

The test Makefile now also tests that perl was indeed found and that the
PERL variable points to an executable before it tries to run the main
test perl script runtests.pl,

Otherwise curl would have to guess where the body ends.

It's against the spec and caused test failures when header
and response were read from the network separately in which
case bug #39 wasn't triggered.

Added an example for demonstrating the usage of curl multi interface
with boost::asio in c++