nhrpd: implement next hop resolution protocol

This provides DMVPN support and integrates to strongSwan. Please read
README.nhrpd and README.kernel for more details.
diff --git a/SERVICES b/SERVICES
index c69d0c1..0322d45 100644
--- a/SERVICES
+++ b/SERVICES
@@ -18,3 +18,4 @@
 ospfapi		2607/tcp
 isisd		2608/tcp
 pimd		2611/tcp
+nhrpd		2612/tcp