o Assert GIANT_REQUIRED on system maps in _vm_map_lock(),

 o Assert GIANT_REQUIRED on system maps in _vm_map_lock(),
   _vm_map_lock_read(), and _vm_map_trylock().  Submitted by: tegge
 o Remove GIANT_REQUIRED from kmem_alloc_wait() and kmem_free_wakeup().
   (This clears the way for exec_map accesses to move outside of Giant.
   The exec_map is not a system map.)
 o Remove some premature MPSAFE comments.

Reviewed by:	tegge