nsswitch: leverage TLS if available in favour over global locking

nsswitch: leverage TLS if available in favour over global locking

The global locking can lead to deadlocks when using nscd: when processing the
first request in winbind, when we know we call into code that will recurse into
winbind we call winbind_off() which sets an environment variable which is later
checked here in the nsswitch module.

But with nscd in the stack, we don't see the env variable in nsswitch, so when
we try to acquire the global lock again, it is already locked and we deadlock.

By using a thread specific winbindd_context, plus a few other thread local global
variables, we don't need a global lock anymore.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>