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

libexec/dma/dmagent/Makefile

@@ -24,11 +24,6 @@ YFLAGS+=	-i
 CLEANFILES=	aliases_parse.i
 FILES=	mailer.conf
 FILESDIR=	${SHAREDIR}/examples/dma
-.if ${MK_SENDMAIL} == no
-CONFGROUPS=	CONFS MAILER
-MAILERDIR=	/etc/mail
-MAILER=		mailer.conf
-.endif
 
 BINMODE=	2555
 

libexec/rc/rc.conf

@@ -596,7 +596,7 @@ allscreens_kbdflags=""	# Set this kbdcontrol mode for all virtual screens
 mta_start_script="/etc/rc.sendmail"
 			# Script to start your chosen MTA, called by /etc/rc.
 # Settings for /etc/rc.sendmail and /etc/rc.d/sendmail:
-sendmail_enable="NO"	# Run the sendmail inbound daemon (YES/NO).
+sendmail_enable="NONE"	# Run the sendmail inbound daemon (YES/NO).
 sendmail_pidfile="/var/run/sendmail.pid"	# sendmail pid file
 sendmail_procname="/usr/sbin/sendmail"		# sendmail process name
 sendmail_flags="-L sm-mta -bd -q30m" # Flags to sendmail (as a server)