Make unix (local) sockets work without IPv6 enabled

Make unix (local) sockets work without IPv6 enabled

The xsocket_type() function had an optional "family" argument
that was enabled only if IPv6 is enabled. In the case of the
function was called with a valid AF_UNIX argument, and IPv6 is
disabled, this argument was silently ignored.

This patch makes the "family" argument mandatory, while keeping
the old behavior i.e., if AF_UNSPEC is passed, we try first IPv6
(if it's enabled) and fallback to IPv4.

Also I changed all callers of xsocket_type() to reflect its new
interface.

Signed-off-by: Jonh Wendell <jonh.wendell@vexcorp.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>