Add a LIST_HEADER macro

To save us from specifying the `next' and `prev' pointers all the time.

It's not perfect, just a tiny bit better.

src/kike.c

@@ -215,9 +215,7 @@ enum
 
 struct client
 {
-	struct client *next;                ///< The next link in a chain
-	struct client *prev;                ///< The previous link in a chain
-
+	LIST_HEADER (client)
 	struct server_context *ctx;         ///< Server context
 
 	int socket_fd;                      ///< The TCP socket
@@ -310,8 +308,7 @@ enum
 
 struct channel_user
 {
-	struct channel_user *prev;
-	struct channel_user *next;
+	LIST_HEADER (channel_user)
 
 	unsigned modes;
 	char nickname[];