Handle messages with "Resent-xxx:" header lines properly (basically mark

Handle messages with "Resent-xxx:" header lines properly (basically mark
the message as being resent); ignore incoming From: lines if their text
exactly matches our machine-readable address exactly -- this causes the
full name to be added when called from (e.g.) MH.  Both of these mode are
to improve interaction with MH.

SCCS-mr: usr.sbin/sendmail/src/headers.c 201
SCCS-mr: usr.sbin/sendmail/src/version.c 201
SCCS-mr: usr.sbin/sendmail/src/sendmail.h 201
SCCS-mr: usr.sbin/sendmail/src/conf.c 201
SCCS-vsn: usr.sbin/sendmail/src/headers.c 3.53
SCCS-vsn: usr.sbin/sendmail/src/version.c 3.322
SCCS-vsn: usr.sbin/sendmail/src/sendmail.h 3.116
SCCS-vsn: usr.sbin/sendmail/src/conf.c 3.70