xdg: implement a better way

Brief about changes:
- add get_irssi_cache_dir()
- add get_irssi_runtime_dir()
- remove is_xdg_supported()

About the proposed implementation:
- use XDG only when the user has the XDG_CONFIG_HOME/irssi directory
- irssi_dir would equal XDG_DATA_HOME/irssi or ~/.irssi, depending on
whether XDG is enabled or not, respectively.
- if xdg is enabled, variables irssi_cache, irssi_runtime, would equal
their respective XDG paths.
- if xdg is *not* enabled, variables irssi_cache, irssi_runtime, would
equal irssi_dir

Notes for future code:
- functions that use the irssi config file would use get_irssi_config()
- functions that use the irssi data directory would use get_irssi_dir()
- functions that use the irssi cache directory would use
get_irssi_cache_dir()
- functions that use the irssi runtime directory would use
get_irssi_runtime_dir()

Stuff that are still missing for complete implementation:
- add checks to ensure the new referenced directories are present
- docs

src/common.h

@@ -55,9 +55,11 @@ int g_input_add(GIOChannel *source, int condition,
 int g_input_add_full(GIOChannel *source, int priority, int condition,
 		     GInputFunction function, void *data);
 
-/* return if xdg is being used */
-int is_xdg_supported();
-/* return full path for irssi folder */
+/* return full path for irssi runtime directory */
+const char *get_irssi_runtime_dir(void);
+/* return full path for irssi cache directory */
+const char *get_irssi_cache_dir(void);
+/* return full path for irssi data directory */
 const char *get_irssi_dir(void);
 /* return full path for irssi config */
 const char *get_irssi_config(void);