uwrap: Support scenario where threads creates threads.

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

Signed-off-by: Robin Hack <hack.robin@gmail.com>
tests: Add new test_thread_create_thread_setgid.

Test aims case where thread change uid/gid and then create new thread.

Signed-off-by: Robin Hack <hack.robin@gmail.com>