Add command line option -C to specify config directory

Also expand usage message to explain what each option means

include/config/rcxml.h

@@ -11,6 +11,8 @@
 #include "theme.h"
 
 struct rcxml {
+	char *config_dir;
+
 	/* core */
 	bool xdg_shell_server_side_deco;
 	int gap;

include/config/session.h

@@ -4,15 +4,17 @@
 
 /**
  * session_environment_init - set enrivonment variables based on <key>=<value>
+ * @dir: path to config directory
  * pairs in `${XDG_CONFIG_DIRS:-/etc/xdg}/lawbc/environment` with user override
  * in `${XDG_CONFIG_HOME:-$HOME/.config}`
  */
-void session_environment_init(void);
+void session_environment_init(const char *dir);
 
 /**
  * session_autostart_init - run autostart file as shell script
+ * @dir: path to config directory
  * Note: Same as `sh ~/.config/labwc/autostart` (or equivalent XDG config dir)
  */
-void session_autostart_init(void);
+void session_autostart_init(const char *dir);
 
 #endif /* __LABWC_SESSION_H */