dynamically allocate header fields (and more....); miscellaneous

dynamically allocate header fields (and more....); miscellaneous
cleanup (mostly comments)

SCCS-vsn: usr.sbin/sendmail/src/headers.c 5.27
SCCS-vsn: usr.sbin/sendmail/src/usersmtp.c 5.26
SCCS-vsn: usr.sbin/sendmail/src/conf.h 5.31
SCCS-vsn: usr.sbin/sendmail/src/domain.c 5.36
SCCS-vsn: usr.sbin/sendmail/src/collect.c 5.16
SCCS-vsn: usr.sbin/sendmail/src/version.c 5.131
SCCS-vsn: usr.sbin/sendmail/src/daemon.c 5.54
SCCS-vsn: usr.sbin/sendmail/src/util.c 5.34
SCCS-vsn: usr.sbin/sendmail/src/readcf.c 5.52
SCCS-vsn: usr.sbin/sendmail/src/queue.c 5.51
SCCS-vsn: usr.sbin/sendmail/src/alias.c 5.41