arrange to be able to define options; put precedences in .cf file;

arrange to be able to define options; put precedences in .cf file;
send errors to an Errors-To: field; fix a serious bug causing mail
to not be delivered to anyone if there were any errors.

SCCS-vsn: usr.sbin/sendmail/src/main.c 3.115
SCCS-vsn: usr.sbin/sendmail/src/conf.c 3.57
SCCS-vsn: usr.sbin/sendmail/src/deliver.c 3.116
SCCS-vsn: usr.sbin/sendmail/src/sendmail.h 3.88
SCCS-vsn: usr.sbin/sendmail/src/savemail.c 3.44
SCCS-vsn: usr.sbin/sendmail/src/version.c 3.204
SCCS-vsn: usr.sbin/sendmail/src/readcf.c 3.32
SCCS-vsn: usr.sbin/sendmail/src/headers.c 3.35