implement arpatounix; log more info; allow nested $?...$|...$.;

implement arpatounix; log more info; allow nested $?...$|...$.;
POSIX compatibility; handle SMTP MAIL command errors properly;
do early diagnosis of .cf errors; adjust checkpointing; misc bugs

SCCS-vsn: usr.sbin/sendmail/src/headers.c 6.4
SCCS-vsn: usr.sbin/sendmail/src/macro.c 6.2
SCCS-vsn: usr.sbin/sendmail/src/main.c 6.7
SCCS-vsn: usr.sbin/sendmail/src/envelope.c 6.2
SCCS-vsn: usr.sbin/sendmail/src/arpadate.c 6.2
SCCS-vsn: usr.sbin/sendmail/src/version.c 6.10
SCCS-vsn: usr.sbin/sendmail/src/util.c 6.3
SCCS-vsn: usr.sbin/sendmail/src/readcf.c 6.4
SCCS-vsn: usr.sbin/sendmail/src/deliver.c 6.9
SCCS-vsn: usr.sbin/sendmail/src/queue.c 6.4