s4-lib/socket: Return the original EMSGSIZE when sendto() and setsockopt() both fail

s4-lib/socket: Return the original EMSGSIZE when sendto() and setsockopt() both fail

This ensures that should we be unable to increase the socket size, we return an
error that the application layer above might expect and be able to make
as reasonable response to (such as switching to a stream-based transport).

This fixes up c692bb02b039ae8fef6ba968fd13b36ad7d62a72.

As suggested by metze in https://bugzilla.samba.org/show_bug.cgi?id=9697#c4

Andrew Bartlett

Reviewed-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit 50b42d1c5bb19e3a5050d7d23ac96e273d3974ee)