fix and optimize non-default-type mutex behavior

problem 1: mutex type from the attribute was being ignored by
pthread_mutex_init, so recursive/errorchecking mutexes were never
being used at all.

problem 2: ownership of recursive mutexes was not being enforced at
unlock time.

src/internal/pthread_impl.h

@@ -47,6 +47,7 @@ struct pthread {
 #define _m_lock __u.__i[1]
 #define _m_waiters __u.__i[2]
 #define _m_owner __u.__i[3]
+#define _m_count __u.__i[4]
 #define _c_block __u.__i[0]
 #define _c_clock __u.__i[1]
 #define _rw_wrlock __u.__i[0]