runtests.pl: fix startup of IPv6 servers

Commit curl-7_23_1-143-g8218064 changed the parameter of
responsive_http_server to accept types other than IPv6 (converting
from a boolean to a string), but only considered the lower-case "ipv6"
and not the "IPv6" variant. This caused all servers to start in IPv4
mode instead.

This patch converts the remaining cases to "ipv6". While not strictly
necessary for the run*server variants, these got also converted for
consistency and to prevent future errors.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
lib/connect: restrict IP/TCP options to said sockets

This patch prepares for adding UNIX domain sockets support.

TCP_NODELAY and TCP_KEEPALIVE are specific to TCP/IP sockets, so do not
apply these to other socket types. bindlocal only works for IP sockets
(independent of TCP/UDP), so filter that out too for other types.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
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. An enum is used here with
switch to avoid compiler warnings in connect_to, complaining that rc
is possibly undefined (which is not possible as socket_domain is
always set).

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) is not considered nor tested. It does not make
sense anyway, first connecting through a TCP proxy, then let that TCP
proxy connect to a UNIX socket.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
sws: try to remove socket and retry bind

If sws is killed it might leave a stale socket file on the filesystem
which would cause an EADDRINUSE error. After this patch, it is checked
whether the socket is really stale and if so, the socket file gets
removed and another bind is executed.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
tests: add HTTP UNIX socket server testing support

The variable `$ipvnum` can now contain "unix" besides the integers 4
and 6 since the variable. Functions which receive this parameter
have their `$port` parameter renamed to `$port_or_path` to support a
path to the UNIX domain socket (as a "port" is only meaningful for TCP).

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
tests: add two HTTP over UNIX socket tests

test1435: a simple test that checks whether a HTTP request can be
performed over the UNIX socket. The hostname/port are interpreted
by sws and should be ignored by cURL.

test1436: test for the ability to do two requests to the same host,
interleaved with one to a different hostname.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
libcurl: 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 and skipping proxies).

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>