* thread.h (struct pthread_rwlock::RWLOCK_READER): Add counter n.

* thread.cc (pthread_rwlock::rdlock): If a thread already owns a
	read lock, just count the number of locks for it, per SUSv4.
	(pthread_rwlock::tryrdlock): Ditto.
	(pthread_rwlock::unlock): If a thread has more than one concurrent
	read locks, just count down.

winsup/cygwin/thread.h

@@ -556,6 +556,7 @@ public:
   {
     struct RWLOCK_READER *next;
     pthread_t thread;
+    unsigned long n;
   } *readers;
   fast_mutex readers_mx;