fixes to restrict users who don't have a "regular" login shell (i.e.,

fixes to restrict users who don't have a "regular" login shell (i.e.,
not listed in /etc/shells) from forwarding to a program or file; they can
write their .forward file from another machine and then get in.

SCCS-vsn: usr.sbin/sendmail/src/READ_ME 8.40
SCCS-vsn: usr.sbin/sendmail/src/recipient.c 8.31
SCCS-vsn: usr.sbin/sendmail/src/deliver.c 8.58