lib: Add messaging_dgm

Messaging based on unix domain datagram sockets

This makes every process participating in messaging bind on a unix domain
datagram socket, similar to the source4 based messaging. The details are a bit
different though:

Retry after EWOULDBLOCK is done with a blocking thread, not by polling. This
was the only way I could in experiments avoid a thundering herd or high load
under Linux in extreme overload situations like many thousands of processes
sending to one blocked process. If there are better ideas to do this in a
simple way, I'm more than happy to remove the pthreadpool dependency again.

There is only one socket per process, not per task. I don't think that per-task
sockets are really necessary, we can do filtering in user space. The message
contains the destination server_id, which contains the destination task_id. I
think we can rebase the source4 based imessaging on top of this, allowing
multiple imessaging contexts on top of one messaging_context. I had planned to
do this conversion before this goes in, but Jeremy convinced me that this has
value in itself :-)

Per socket we also create a fcntl-based lockfile to allow race-free cleanup of
orphaned sockets. This lockfile contains the unique_id, which in the future
will make the server_id.tdb obsolete.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

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