add PTHREAD_NULL

this is added for POSIX-future as the outcome of Austin Group issue
599. since it's in the reserved namespace for pthread.h, there are no
namespace considerations for adding it early.

include/pthread.h

@@ -74,6 +74,9 @@ extern "C" {
 #define PTHREAD_BARRIER_SERIAL_THREAD (-1)
 
 
+#define PTHREAD_NULL ((pthread_t)0)
+
+
 int pthread_create(pthread_t *__restrict, const pthread_attr_t *__restrict, void *(*)(void *), void *__restrict);
 int pthread_detach(pthread_t);
 _Noreturn void pthread_exit(void *);