Implemented XDG basedir spec for global config

(http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html)
Looks for global config file in:
  * SURFRAW_global_conf if set
  * surfraw/conf under each dir in (colon separated) $XDG_CONFIG_DIRS.
  * @sysconfigdir@/xdg/surfraw/conf
  * @sysconfigdir@/surfraw.conf (for backwards compatibility)