swrap: add basic support for fd-passing via SCM_RIGHTS

swrap: add basic support for fd-passing via SCM_RIGHTS

We only allow passing up to 6 fds in a single sendmsg call,
in order to keep the logic simple.

That's more than enough for Samba's use of fd-passing,
there we only pass a single fd and the fd will be
closed in the sender on success. It means it's ok
to keep the socket_info.io.pck_{snd,rcv} fields per process
and the PCAP generation will still work as expected.

If these constraints turn out to be a problem for other applications,
we need to change to a more complex design and move the socket_info array
into a shared memory file and use shared robust mutexes.

But for now we just want to support multi-channel testing in Samba.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11899

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Anoop C S <anoopcs@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>