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/zebra/client_main.c b/zebra/client_main.c
index 43ab299..75c8867 100644
--- a/zebra/client_main.c
+++ b/zebra/client_main.c
@@ -120,6 +120,7 @@
   { "ospf",   ZEBRA_ROUTE_OSPF },
   { "ospf6",  ZEBRA_ROUTE_OSPF6 },
   { "bgp",    ZEBRA_ROUTE_BGP },
+  { "nhrp",   ZEBRA_ROUTE_NHRP },
   { NULL,     0 }
 };