core: split WeeChat home in 4 directories, use XDG directories by default (issue #1285)

The 4 directories (which can be the same):

- config: configuration files, certificates
- data: log/upgrade files, local plugins, scripts, xfer files
- cache: script repository, scripts downloaded (temporary location)
- runtime: FIFO pipe, relay UNIX sockets

src/plugins/script/script-action.c

@@ -1523,7 +1523,7 @@ script_action_schedule (const char *action,
                         int quiet)
 {
     /* create again "script" directory, just in case it has been removed */
-    if (!weechat_mkdir_home (SCRIPT_PLUGIN_NAME, 0755))
+    if (!weechat_mkdir_home ("${weechat_cache_dir}/" SCRIPT_PLUGIN_NAME, 0755))
         return;
 
     script_action_add (action);