Also reverts commit f254c59d

The Clang compiler found a few implicit conversion problems that have
now been fixed.

Issue: When building a 32bit target with large file support HP-UX
<sys/socket.h> header file may simultaneously provide two different
sets of declarations for sendfile and sendpath functions, one with
static and another with external linkage. Given that we do not use
mentioned functions we really don't care which linkage is the
appropriate one, but on the other hand, the double declaration emmits
warnings when using the HP-UX compiler and errors when using modern
gcc versions resulting in fatal compilation errors.

Mentioned issue is now fixed as long as we don't use sendfile nor
sendpath functions.

Inclusion of top two most included header files now done in setup_once.h

Some HP-UX system headers require TRUE defined to 1 and FALSE to 0.

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.