posix: fix error return code for pthread_mutex_trylock

pthread_mutex_trylock() should return EBUSY if the mutex is already
locked. The translations of CORE_MUTEX_STATUS_NESTING_NOT_ALLOWED is
EDEADLK which is correct for pthread_mutex_lock(). This fixes the
translation for trylock.

Closes #2170.

cpukit/posix/src/mutextrylock.c

@@ -37,5 +37,8 @@ int pthread_mutex_trylock(
   pthread_mutex_t           *mutex
 )
 {
-  return _POSIX_Mutex_Lock_support( mutex, false, THREAD_QUEUE_WAIT_FOREVER );
+  int r = _POSIX_Mutex_Lock_support( mutex, false, THREAD_QUEUE_WAIT_FOREVER );
+  if ( r == EDEADLK )
+    r = EBUSY;
+  return r;
 }

doc/posix_users/mutex.t

@@ -580,8 +580,8 @@ The mutex has the protocol attribute of PTHREAD_PRIO_PROTECT and the
 priority of the calling thread is higher than the current priority
 ceiling.
 
-@item EDEADLK
-The current thread already owns the mutex.
+@item EBUSY
+The mutex is already locked.
 
 @end table