preliminary hacks for multiple SMTP transactions per connection: make

preliminary hacks for multiple SMTP transactions per connection: make
assignment of multiple queue id's more efficient, make prefixes two
characters (e.g., qfAA99999), pass the file name to freeze and thaw,
add a mailer flag saying we are talking to one of our own kind.

SCCS-mr: usr.sbin/sendmail/src/main.c 026
SCCS-mr: usr.sbin/sendmail/src/sendmail.h 026
SCCS-mr: usr.sbin/sendmail/src/version.c 026
SCCS-mr: usr.sbin/sendmail/src/srvrsmtp.c 026
SCCS-mr: usr.sbin/sendmail/src/usersmtp.c 026
SCCS-mr: usr.sbin/sendmail/src/readcf.c 026
SCCS-vsn: usr.sbin/sendmail/src/main.c 3.138
SCCS-vsn: usr.sbin/sendmail/src/sendmail.h 3.100
SCCS-vsn: usr.sbin/sendmail/src/version.c 3.250
SCCS-vsn: usr.sbin/sendmail/src/srvrsmtp.c 3.36
SCCS-vsn: usr.sbin/sendmail/src/usersmtp.c 3.26
SCCS-vsn: usr.sbin/sendmail/src/readcf.c 3.45