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/common.c

@@ -254,7 +254,9 @@ xstrndup (const char *s, size_t n)
 
 // --- Double-linked list helpers ----------------------------------------------
 
-// The links of the list need to have the members `prev' and `next'.
+#define LIST_HEADER(type)                                                      \
+	struct type *next;                                                         \
+	struct type *prev;
 
 #define LIST_PREPEND(head, link)                                               \
 	BLOCK_START                                                                \
@@ -562,8 +564,7 @@ error_propagate (struct error **destination, struct error *source)
 
 struct str_map_link
 {
-	struct str_map_link *next;          ///< The next link in a chain
+	LIST_HEADER (str_map_link)
-	struct str_map_link *prev;          ///< The previous link in a chain
 
 	void *data;                         ///< Payload
 	size_t key_length;                  ///< Length of the key without '\0'

src/kike.c

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

src/zyklonb.c

@@ -52,9 +52,7 @@ static struct config_item g_config_table[] =
 
 struct plugin_data
 {
-	struct plugin_data *next;           ///< The next link in a chain
+	LIST_HEADER (plugin_data)
-	struct plugin_data *prev;           ///< The previous link in a chain
-
 	struct bot_context *ctx;            ///< Parent context
 
 	pid_t pid;                          ///< PID of the plugin process