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/netlink.h b/nhrpd/netlink.h
new file mode 100644
index 0000000..f05596b
--- /dev/null
+++ b/nhrpd/netlink.h
@@ -0,0 +1,24 @@
+/* NHRP netlink/neighbor table API
+ * Copyright (c) 2014-2015 Timo Teräs
+ *
+ * This file is free software: you may copy, redistribute and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+#include <stdint.h>
+
+union sockunion;
+struct interface;
+
+extern int netlink_nflog_group;
+extern int netlink_req_fd;
+
+int netlink_init(void);
+int netlink_configure_arp(unsigned int ifindex, int pf);
+void netlink_update_binding(struct interface *ifp, union sockunion *proto, union sockunion *nbma);
+void netlink_set_nflog_group(int nlgroup);
+
+void netlink_gre_get_info(unsigned int ifindex, uint32_t *gre_key, unsigned int *link_index, struct in_addr *saddr);
+void netlink_gre_set_link(unsigned int ifindex, unsigned int link_index);