WIP: Add BFD support

At this point, this is just an example.  The following issues need to be
resolved before this could be merged:

* Is BFD going to be enabled everywhere?  If not, it needs an option.
  Is that option on or off by default?  If on, should it be set to off
  for existing Routers on migration?
* Should the interval be customizable?  Should the multiplier?

resources/views/api/v4/router/server/bird2/header.foil.php

@@ -60,6 +60,17 @@
 # ignore interface up/down events
 protocol device { }
 
+protocol bfd
+{
+        accept ipv<?= $t->router->protocol ?> direct;
+        interface "*" {
+                passive on;
+                multiplier 3;
+                min rx interval 500ms;
+                min tx interval 500ms;
+        };
+}
+
 # This function excludes weird networks
 #  rfc1918, class D, class E, too long and too short prefixes
 function avoid_martians()

resources/views/api/v4/router/server/bird2/neighbors.foil.php

@@ -284,6 +284,7 @@
 <?php if( $int['bgpmd5secret'] && !$t->router->skip_md5 ): ?>
         password "<?= $int['bgpmd5secret'] ?>";
 <?php endif; ?>
+        bfd on;
 }
 
 <?php endforeach; ?>