vpn: let's get progressive and remove mpd4

This version of MPD was released in 2008 and hasn't received updates
since and has eventually been removed from the FreeBSD ports tree in
2011.  It was replaced by MPD 5, which we also have running with the
latest version from 2016.  If there should be any splatter, we'll fix
it in place.  Progress is healthy.  :)

Makefile

@@ -65,7 +65,6 @@ CORE_DEPENDS?=		apinger \
 			lighttpd \
 			minicron \
 			miniupnpd \
-			mpd4 \
 			mpd5 \
 			ngattach \
 			ntp \

src/etc/inc/plugins.inc.d/vpn.inc

@@ -304,7 +304,7 @@ EOD;
       /* fixed to WAN elsewhere, no need to extend, but at least make it work */
       legacy_netgraph_attach(get_real_interface('wan'));
 
-      mwexec('/usr/local/sbin/mpd4 -b -d /var/etc/pptp-vpn -p /var/run/pptp-vpn.pid -s pptps pptps');
+      mwexec('/usr/local/sbin/mpd5 -b -d /var/etc/pptp-vpn -p /var/run/pptp-vpn.pid -s pptps pptps');
 
       break;
 
@@ -543,7 +543,7 @@ EOD;
 
       legacy_netgraph_attach($pppoe_interface);
 
-      mwexec("/usr/local/sbin/mpd4 -b -d /var/etc/pppoe{$pppoecfg['pppoeid']}-vpn -p /var/run/pppoe{$pppoecfg['pppoeid']}-vpn.pid -s poes poes");
+      mwexec("/usr/local/sbin/mpd5 -b -d /var/etc/pppoe{$pppoecfg['pppoeid']}-vpn -p /var/run/pppoe{$pppoecfg['pppoeid']}-vpn.pid -s poes poes");
 
       break;
   }
@@ -732,7 +732,7 @@ EOD;
 
       legacy_netgraph_attach(get_real_interface($l2tpcfg['interface']));
 
-      mwexec('/usr/local/sbin/mpd4 -b -d /var/etc/l2tp-vpn -p /var/run/l2tp-vpn.pid -s l2tps l2tps');
+      mwexec('/usr/local/sbin/mpd5 -b -d /var/etc/l2tp-vpn -p /var/run/l2tp-vpn.pid -s l2tps l2tps');
 
       break;