Add $[ and $] as RHS operators to look up the contents and pass them

Add $[ and $] as RHS operators to look up the contents and pass them
to maphostname; maphostname currently looks them up in /etc/hosts and
converts them to canonical form, but could be turned into a general
name server.....   huzzah!!

SCCS-vsn: usr.sbin/sendmail/src/headers.c 4.5
SCCS-vsn: usr.sbin/sendmail/src/main.c 4.14
SCCS-vsn: usr.sbin/sendmail/src/daemon.c 4.11
SCCS-vsn: usr.sbin/sendmail/src/version.c 4.39
SCCS-vsn: usr.sbin/sendmail/src/envelope.c 4.7
SCCS-vsn: usr.sbin/sendmail/src/parseaddr.c 4.9
SCCS-vsn: usr.sbin/sendmail/src/readcf.c 4.8