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/nhrpd/reqid.c b/nhrpd/reqid.c
new file mode 100644
index 0000000..24b3199
--- /dev/null
+++ b/nhrpd/reqid.c
@@ -0,0 +1,49 @@
+#include "zebra.h"
+#include "hash.h"
+#include "nhrpd.h"
+
+static unsigned int nhrp_reqid_key(void *data)
+{
+	struct nhrp_reqid *r = data;
+	return r->request_id;
+}
+
+static int nhrp_reqid_cmp(const void *data, const void *key)
+{
+	const struct nhrp_reqid *a = data, *b = key;
+	return a->request_id == b->request_id;
+}
+
+uint32_t nhrp_reqid_alloc(struct nhrp_reqid_pool *p, struct nhrp_reqid *r, void (*cb)(struct nhrp_reqid *, void *))
+{
+	if (!p->reqid_hash) {
+		p->reqid_hash = hash_create(nhrp_reqid_key, nhrp_reqid_cmp);
+		p->next_request_id = 1;
+	}
+
+	if (r->cb != cb) {
+		r->request_id = p->next_request_id;
+		if (++p->next_request_id == 0) p->next_request_id = 1;
+		r->cb = cb;
+		hash_get(p->reqid_hash, r, hash_alloc_intern);
+	}
+	return r->request_id;
+}
+
+void nhrp_reqid_free(struct nhrp_reqid_pool *p, struct nhrp_reqid *r)
+{
+	if (r->cb) {
+		hash_release(p->reqid_hash, r);
+		r->cb = NULL;
+	}
+}
+
+struct nhrp_reqid *nhrp_reqid_lookup(struct nhrp_reqid_pool *p, uint32_t reqid)
+{
+	struct nhrp_reqid key;
+	if (!p->reqid_hash) return 0;
+	key.request_id = reqid;
+	return hash_lookup(p->reqid_hash, &key);
+}
+
+