separate support for header vs envelope sender; if users are in the

separate support for header vs envelope sender; if users are in the
database, map them to a canonical name (e.g., @CS.Berkeley.EDU) in header.

SCCS-vsn: usr.sbin/sendmail/src/usersmtp.c 5.17
SCCS-vsn: usr.sbin/sendmail/src/envelope.c 5.24
SCCS-vsn: usr.sbin/sendmail/src/err.c 5.12
SCCS-vsn: usr.sbin/sendmail/src/version.c 5.85
SCCS-vsn: usr.sbin/sendmail/src/sendmail.h 5.26
SCCS-vsn: usr.sbin/sendmail/src/savemail.c 5.15
SCCS-vsn: usr.sbin/sendmail/src/srvrsmtp.c 5.32
SCCS-vsn: usr.sbin/sendmail/src/deliver.c 5.51
SCCS-vsn: usr.sbin/sendmail/src/udb.c 5.15