mail: make The Dragonfly Mail Agent (dma) the default mta.

dma accepts mail from a local Mail User Agent (MUA) and delivers it
locally or to a smarthost for delivery. dma does not accept inbound
mail (i.e., it does not listen on port 25) and is not intended to
provide the same functionality as a full MTA like postfix or sendmail.
It is intended for use cases such as delivering cron(8) mail. which
is the default configuration and usage of sendmail in the default
setup of the base system.

In order to switch the default from sendmail to dma, we teach
mailwrapper to fallback on dma directly if the mailer.conf file cannot
be opened.
We install by default a mailer.conf file which points at dma
We install a mailer.conf file for sendmail in the examples.

Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D37035