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.c

@@ -371,7 +371,7 @@ weechat_plugin_init (struct t_weechat_plugin *plugin, int argc, char *argv[])
 
     script_config_read ();
 
-    weechat_mkdir_home (SCRIPT_PLUGIN_NAME, 0755);
+    weechat_mkdir_home ("${weechat_cache_dir}/" SCRIPT_PLUGIN_NAME, 0755);
 
     script_command_init ();
     script_completion_init ();