s3:prefork: Allow to associate private data with listening socket

s3:prefork: Allow to associate private data with listening socket

Prepare for merger RPC server codebase, where it will be necessary to
stablish an association between the listening socket and the
dcesrv_endpoint that the socket is serving.

Signed-off-by: Samuel Cabrero <scabrero@suse.de>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>