Switching from sendmail to Dragonfly Mail Agent by default

Hello everyone,

As of today, on a default FreeBSD setup, a mailer agent is configured in order
to be able to distribute locally emails (from crontab for example) and/or for
relaying those emails. This role has been served by a stripped down version of
sendmail up to now. By stripped down, I mean it is built without the support
for feature that would make it a full featured MTA, like no support for ldap.

Long time ago we have imported Dragonfly Mail Agent, a minimalistic MTA born
within the Dragonfly Project, covering exactly those needs and only those.

It has matured slowly over the time and we believe we have addressed all the
major issues reported preventing it from being the default.

For FreeBSD 14 we would like to activate it by default.

It means:
- install by default mailer.conf from dma (and install the one from sendmail
  in /usr/share/example/sendmail)
- activate sendmail_enable=NONE by default in /etc/default/rc.conf
- make mailwrappe fallback on dma.

If noone brings an obvious blocker, this change will happen in the next couple
of weeks!

Best regards,
Bapt