async_req: make async_connect_send() "reentrant"

async_req: make async_connect_send() "reentrant"

Allow callers to pass in socket fds that where already passed to an
earlier call of async_connect_send(). Callers expect this behaviour and
it was working until 05d4dbda8357712cb81008e0d611fdb0e7239587 broke it.

The proper fix would be to change callers to close the fd and start from
scratch with a fresh socket.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12105

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Thu Aug  4 05:03:21 CEST 2016 on sn-devel-144

(cherry picked from commit 9c6a4ea2788808bdcc7bfea798d838ea56c3b5ec)