unix_msg: always create a send queue for a peer

unix_msg: always create a send queue for a peer

Previously, we only created a send queue for a peer if the initial send
to the non-blocking non-connected socket reported EWOULDBOCK (because
the channel was full).

With this change, we now always create a send queue and use a connected,
non-blocking datagram socket from the beginning.

Initially, the socket of the send queue is set to non-blocking mode and
we attempt a direct send via sendmsg(). If that returns EWOULDBOCK, we
set the send queue to blocking mode and let the threadpool handle the
IO.

When a send queue becomes empty, we set the send queue socket back to
non-blocking.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>