The code for recursive mutexes triggered Undefined Behavior
and crash as pthread_equal(3) was not used in a portable way.
On the first call of mutex_lock() pthread_equal() was called with
a pthread_t of value -1 which is invalid.
Passing a non valid thread ID to pthread_equal is undefined.
Switch the backend to native interface for POSIX recursive
mutexes with PTHREAD_MUTEX_RECURSIVE and simplify the code.
Detected on NetBSD/amd64 9.99.46.
Signed-off-by: Marvin Scholz <epirat07@gmail.com>
bc2d0d8c41