Add context versions of wbclient functions

Add context versions of wbclient functions

To make the libwbclient library thread-safe, all functions
that call through to wb_common winbindd_request_response need
to have context that they can use. This commit adds all the
necessary functions.

Signed-off-by: Matthew Newton <matthew-git@newtoncomputing.co.uk>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>