Fix: improve Unix socket passing detection

This commit adds the support for the torsocks recvmsg wrapper to detect
multiple FDs being passed through a Unix socket.

Furthermore, we now don't exit anymore but simply fire a debug message
and return EACCES to the caller.

Finally, a test is added for inet socket passing detection called
test_fd_passing.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>