tdb_wrap: Make mutexes easier to use

tdb_wrap: Make mutexes easier to use

This patch makes sure we work fine and open without mutexes if they
are not supported.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>
(cherry picked from commit 911439639a2caa17a9680cb297e9fe26eba56149)

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11032

Autobuild-User(v4-2-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-2-test): Mon Jan 12 22:28:31 CET 2015 on sn-devel-104