Bring DragonFly's sendmail infrastructure up to date. This commit includes:

Bring DragonFly's sendmail infrastructure up to date.  This commit includes:

- Upgrading base system from sendmail 8.12.10 to 8.13.4 (which includes
  native DragonFly support so DragonFly no longer needs to pretend to be
  FreeBSD).
- Drop the FreeBSD-only add-ons of nobiff and nofsync for mail.local
  and the FreeBSD-customized Received: header format and instead use the
  distribution as released by sendmail.org.
- Default to /etc/mail/dragonfly.mc and /etc/mail/dragonfly.submit.mc
  instead of /etc/mail/freebsd{,.submit}.mc.
  - Users should be sure to 'cd /usr/src; make upgrade' to get the
    new dragonfly{,.submit}.mc files installed into /etc/mail/.
    (Or you can just do the sendmail portion with 'cd /usr/src/etc/sendmail;
    make upgrade'.)
- Support for per-host submit.mc files (i.e., /etc/mail/`hostname`.submit.mc).
- New SENDMAIL_CF_DIR make.conf variable for overriding location of the
  sendmail/cf directory (default is still /usr/share/sendmail/cf/).
- Better sendmail_enable checking in /etc/rc.sendmail.
- /etc/periodic/daily/150.clean-hoststat now does the right thing and only
  cleans expired host status entries using the purgestat command.