add "envelopes" to contain the basic information needed as control

add "envelopes" to contain the basic information needed as control
info for each message.  currently there is only one envelope -- this
being the obvious stupid conversion.  later there will be separate
envelopes for error messages, return receipts, etc.

SCCS-vsn: usr.sbin/sendmail/src/alias.c 3.31
SCCS-vsn: usr.sbin/sendmail/src/conf.c 3.46
SCCS-vsn: usr.sbin/sendmail/src/deliver.c 3.79
SCCS-vsn: usr.sbin/sendmail/src/collect.c 3.37
SCCS-vsn: usr.sbin/sendmail/src/err.c 3.20
SCCS-vsn: usr.sbin/sendmail/src/sendmail.h 3.67
SCCS-vsn: usr.sbin/sendmail/src/parseaddr.c 3.41
SCCS-vsn: usr.sbin/sendmail/src/savemail.c 3.28
SCCS-vsn: usr.sbin/sendmail/src/version.c 3.104
SCCS-vsn: usr.sbin/sendmail/src/main.c 3.74
SCCS-vsn: usr.sbin/sendmail/src/recipient.c 3.35
SCCS-vsn: usr.sbin/sendmail/src/srvrsmtp.c 3.15
SCCS-vsn: usr.sbin/sendmail/src/headers.c 3.18
SCCS-vsn: usr.sbin/sendmail/src/queue.c 3.13