only clear bits specified in perm, or, if setting as well, clear all in group.

only clear bits specified in perm, or, if setting as well, clear all in group.
deal with "g=" vs. "g=u" bogon.  update commands if:
(1) perm set and we get a 'permcopy'
(2) perm set and nothing left to do
(3) nothing left to do and we've there's an 'outstanding ='
fix umask handling for '-' when 'who' is unset.
condense code for '='.
From: "Chris G. Demetriou" <cgd@postgres.Berkeley.EDU>

SCCS-vsn: lib/libc/gen/setmode.c 8.2