uwrap: Support scenario where threads fork or creates threads.

uwrap: Support scenario where threads fork or creates threads.

When fork() is called here there is no need to disable uwrap as a whole.
This change disables only uwrap for the thread which called fork().

uwrap catches calls of pthread_create() and pthread_exit() functions
from libpthread library now.

Pair-Programmed-With: Andreas Schneider <asn@samba.org>
Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Robin Hack <hack.robin@gmail.com>
Signed-off-by: Andreas Schneider <asn@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>