stream-unix: only use path-based socket names

stream-unix: only use path-based socket names

FreeBSD returns a socklen of sockaddr_storage when doing an accept on an unix
STREAM socket. The current code will assume it means a sun_path larger than 0.

That breaks some tests like the one below which don't expect to find "unix::" on
the logs.

As a Linux abstract address would not have a more useful name either, it's
better to check that sun_path starts with a non-zero byte and return 0 length in
case it doesn't.

402: ovs-ofctl replace-flows with --bundle      FAILED (ovs-ofctl.at:2928)
2016-07-08T12:44:30.068Z|00020|vconn|DBG|unix:: sent (Success): OFPT_HELLO (OF1.6) (xid=0x1):

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>