Request exclusive access to resource

        Entry:
          AH = 12h
          BH = 14h
          BL = object
               00h mailbox handle is DWORD on top of stack
               02h use given task's mailbox (task's handle on top of stack)
               03h use current task's default mailbox

        Note:   Release exclusive access by sending CLOSE message to
                mailbox access may be requested multiple times, and
                requires multiple CLOSEs