pkgbase: resolve mailer.conf conflict WITHOUT_SENDMAIL

When WITHOUT_SENDMAIL is set, we end up with two different mailer.conf that
conflict, and hilarity ensues. There's currently three different places that
we might install mailer.conf:

- ^/etc/Makefile (package=runtime, contingent on MK_MAIL != no)
- ^/libexec/dma/dmagent/Makefile (package=dma, contingent on MK_SENDMAIL !=
no)
- ^/usr.sbin/mailwrapper/Makefile (package=utilities, contingent on
not-installed)

The mailwrapper installation will effectively never happen because the ^/etc
one will first.

This patch simplifies the whole situation; remove the ^/etc/Makefile version
and install it primarily in mailwrapper if MK_MAILWRAPPER != "no". The
scenarios covered in mailwrapper are:

- sendmail(8) is installed, dma(8) may or may not be installed
- neither sendmail(8) nor dma(8) is installed

In the first scenario, sendmail(8) is dominant so we can go ahead and
install the version in ^/etc/mail. In the unlisted scenario, sendmail(8) is
not installed but dma(8) is, we'll let ^/libexec/dma/dmagent do the
installation. In the second listed scenario, we still want to install an
example mailer.conf so just install the base sendmail(8) version.

Reviewed by:	bapt
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D24924

etc/Makefile

@@ -17,11 +17,10 @@ SUBDIR+=sendmail
 
 # NB: keep these sorted by MK_* knobs
 
-.if ${MK_SENDMAIL} == "no"
+ETCMAIL=aliases
-ETCMAIL=mailer.conf aliases
+.if ${MK_SENDMAIL} != "no"
-.else
+ETCMAIL+=Makefile README access.sample virtusertable.sample \
-ETCMAIL=Makefile README mailer.conf access.sample virtusertable.sample \
+	mailertable.sample
-	mailertable.sample aliases
 .endif
 
 # Special top level files for FreeBSD

etc/mail/mailer.conf

@@ -2,6 +2,9 @@
 #
 # Execute the "real" sendmail program, named /usr/libexec/sendmail/sendmail
 #
+# If dma(8) is installed, an example mailer.conf that uses dma(8) instead can
+# can be found in /usr/share/examples/dma.
+#
 sendmail	/usr/libexec/sendmail/sendmail
 mailq		/usr/libexec/sendmail/sendmail
 newaliases	/usr/libexec/sendmail/sendmail

usr.sbin/mailwrapper/Makefile

@@ -26,7 +26,11 @@ SYMLINKS+=	..${BINDIR}/mailwrapper /bin/rmail
 .endif
 
 .if ${MK_MAILWRAPPER} != "no"
-.if !exists(${DESTDIR}/etc/mail/mailer.conf)
+# We install here if either sendmail(8) is enabled, or dma(8) isn't.  In the
+# latter scenario, we take care of the possibility that neither sendmail(8) nor
+# dma(8) are installed and simply provide a default that can be changed for an
+# alternative in ports.
+.if ${MK_SENDMAIL} != "no" || ${MK_DMAGENT} == "no"
 FILES=		${SRCTOP}/etc/mail/mailer.conf
 FILESDIR=	/etc/mail
 FILESMODE=	644