Adding support for correct handling of pthread_mutex_trylock() on Win32
systems as well as when using the safe mutexes. 


include/my_pthread.h:
  Adding win_pthread_mutex_trylock() function as wrapper function for
  Windows implementation of pthread_mutex_trylock().
  
  Adding flag to safe_mutex_lock() that is shall not abort if the mutex
  is already locked and instead return EBUSY since this is the POSIX
  behaviour.
mysys/my_winthread.c:
  Added Win32 wrappper function to handle pthread_mutex_trylock().
mysys/thr_mutex.c:
  Added parameter 'try_lock' to safe_mutex_lock() to allow it to do double-
  duty as a pthread_mutex_trylock() in addition to working as
  pthread_mutex_lock().
  
  The code needs to return EBUSY instead of throwing an error in the event
  that the mutex is re-locked (we do not have recursive mutexes), but it
  also requires some special handling to avoid race conditions when
  calling the real pthread_mutex_{lock,trylock}().