sws: move away from IPv4/IPv4-only assumption

Instead of depending the socket domain type on use_ipv6, specify the
domain type (AF_INET / AF_INET6) as variable.

Besides abstracting the socket type, make the debugging messages be
independent on IP (introduce location_str which points to "port XXXXX").
Rename "ipv_inuse" to "socket_type" and tighten the scope (main).

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
sws: restrict TCP_NODELAY to IP sockets

TCP_NODELAY does not make sense for Unix sockets, so enable it only if
the socket is using IP.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
sws: add UNIX domain socket support

This extends sws with a --unix-socket option which causes the port to
be ignored (as the server now listens on the path specified by
--unix-socket). This feature will be available in the following patch
that enables checking for UNIX domain socket support.

Proxy support (CONNECT) was not considered nor tested.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Add UNIX domain sockets support

The ability to do HTTP requests over a UNIX domain socket has been
requested before, in Apr 2008 [0][1] and Sep 2010 [2]. While a
discussion happened, no patch seems to get through. I decided to give it
a go since I need to test a nginx HTTP server which listens on a UNIX
domain socket.

One patch [3] seems to make it possible to use the
CURLOPT_OPENSOCKETFUNCTION function to gain a UNIX domain socket.
Another person wrote a Go program which can do HTTP over a UNIX socket
for Docker[4] which uses a special URL scheme (though the name contains
cURL, it has no relation to the cURL library).

This patch considers support for UNIX domain sockets at the same level
as HTTP proxies / IPv6, it acts as an intermediate socket provider and
not as a separate protocol. Since this feature affects network
operations, a new feature flag was added ("unix-sockets") with a
corresponding CURL_VERSION_UNIX_SOCKETS macro.

A new CURLOPT_UNIX_SOCKET_PATH option is added and documented. This
option enables UNIX domain sockets support for all requests on the
handle (replacing IP sockets).

A new configure option (--enable-unix-sockets) and CMake option
(ENABLE_UNIX_SOCKETS) can disable this optional feature. Note that I
deliberately did not mark this feature as advanced, this is a
feature/component that should easily be available.

 [0]: http://curl.haxx.se/mail/lib-2008-04/0279.html
 [1]: http://daniel.haxx.se/blog/2008/04/14/http-over-unix-domain-sockets/
 [2]: http://sourceforge.net/p/curl/feature-requests/53/
 [3]: http://curl.haxx.se/mail/lib-2008-04/0361.html
 [4]: https://github.com/Soulou/curl-unix-socket

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
tool: add --unix-socket option

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
tests: move TESTCASES to Makefile.inc, add show for cmake

This change allows runtests.pl to be run from the CMake builddir:

    export srcdir=/tmp/curl/tests;
    perl -I$srcdir $srcdir/runtests.pl -l

In order to make this possible, all test cases have been moved from
Makefile.am to Makefile.inc.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>