[project @ 1999-03-17 16:28:34 by sewardj]

Make mut_link field be NULL when removing CAF_{UN}ENTERED from
mut_once_list.  Convention is to have the field be NULL iff
the caf is not on a mut_once_list.  This gives a O(1) cost way
to avoid putting a caf onto a mut_once_list more than once.