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/zebra_rib.c b/zebra/zebra_rib.c
index 2aecf5d..9ca0290 100644
--- a/zebra/zebra_rib.c
+++ b/zebra/zebra_rib.c
@@ -72,6 +72,7 @@
   [ZEBRA_ROUTE_ISIS]    = {ZEBRA_ROUTE_ISIS,    115},
   [ZEBRA_ROUTE_BGP]     = {ZEBRA_ROUTE_BGP,      20  /* IBGP is 200. */},
   [ZEBRA_ROUTE_BABEL]   = {ZEBRA_ROUTE_BABEL,    95},
+  [ZEBRA_ROUTE_NHRP]    = {ZEBRA_ROUTE_NHRP,     10},
   /* no entry/default: 150 */
 };
 
@@ -1423,6 +1424,7 @@
   [ZEBRA_ROUTE_BGP]     = 3,
   [ZEBRA_ROUTE_HSLS]    = 4,
   [ZEBRA_ROUTE_BABEL]   = 2,
+  [ZEBRA_ROUTE_NHRP]    = 2,
 };
 
 /* Look into the RN and queue it into one or more priority queues,