Description

   This routine attempts to acquire a read-write lock for write
   access. The routine attempts to immediately acquire the lock.
   If the lock is acquired, zero (0) is returned. If the lock is
   held by another thread (for either read or write access), [EBUSY]
   is returned and the calling thread does not wait for the write-
   access lock to be acquired.

   Note that it is a coding error to attempt to acquire the lock
   for write access if the lock is already held by the calling
   thread. (However, this routine returns [EBUSY] anyway, because
   no ownership error-checking takes place.)