Add parsing rules for IPv4/IPv6 subnet literal constants, addresses #888

This fixes specifying IPv4 subnets in IPv4-mapped-IPv6 format with a
mask length relative to the 128 bits of the mapped IPv6 address.

src/util.h

@@ -91,6 +91,9 @@ void delete_each(T* t)
 		delete *it;
 	}
 
+std::string extract_ip(const std::string& i);
+std::string extract_ip_and_len(const std::string& i, int* len);
+
 std::string get_unescaped_string(const std::string& str);
 std::string get_escaped_string(const std::string& str, bool escape_all);