Move wbc global variables into global context instead

Move wbc global variables into global context instead

There are some global variables in use in the libwbclient
library. Now that we have a context, move these into it so that
they are thread-safe when the wbcCtx* functions are used.

https://bugzilla.samba.org/show_bug.cgi?id=11149

Signed-off-by: Matthew Newton <matthew-git@newtoncomputing.co.uk>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
(cherry picked from commit 2664d9070f3c21231627380822e619fb695db662)