socket-util: Work around Unix domain socket path name limits on Linux.

socket-util: Work around Unix domain socket path name limits on Linux.

Many Open vSwitch tests fail on Debian's automatic build machines because
the builds occur in deeply nested directories with long names.  OVS tries
to bind and connect to Unix domain sockets using absolute path names, which
in combination with long directory names means that the socket's name
exceeds the limit for Unix domain socket names (108 bytes on Linux).

This commit works around the problem on Linux by indirecting through
/proc/self/fd/<dirfd>/<basename> when names exceed the maximum that can be
used directly.

Reported-by: Hector Oron <hector.oron@gmail.com>
Reported-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reported-by: Roger Leigh <rleigh@codelibre.net>
Debian bug #602891.
Debian bug #602911.