add the 'R' flag to mailers, saying to rewrite the recipient addresses

add the 'R' flag to mailers, saying to rewrite the recipient addresses
to be relative to the recipient.  This makes reply code easy, but
confuses user mail programs that are expecting to have to rewrite
recipient addresses.  In general, the receiving host must be "smart"
for this to work.

SCCS-vsn: usr.sbin/sendmail/src/deliver.c 3.77
SCCS-vsn: usr.sbin/sendmail/src/version.c 3.102
SCCS-vsn: usr.sbin/sendmail/src/readcf.c 3.23
SCCS-vsn: usr.sbin/sendmail/doc/intro/intro.me 3.18