zdtm: test that UNIX socket owner/group don't change

In container c/r(OVZ) UNIX socket uid and gid restored from
connected peer credentials instead of inode once.

https://bugzilla.openvz.org/show_bug.cgi?id=2969

changes:
v2 - cleanup headers, close socket, use err instead fail,
print id's on error.
v3 - wrong patch
v4 - move to sockets00, correct patch-message
v5 - minor cleanup

Signed-off-by: Pavel Tikhomirov <ptikhomirov@parallels.com>
Acked-by: Andrey Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>