Add /etc/xkb as extra lookup path for system data files

Add /etc/xkb as extra lookup path for system data files

This completes the usual triplet of configuration locations available for most
processes:
- vendor-provided data files in /usr/share/X11/xkb
- system-specific data files in /etc/xkb
- user-specific data files in $XDG_CONFIG_HOME/xkb

The default lookup order user, system, vendor, just like everything else that
uses these conventions.

For include directives in rules files, the '%E' resolves to that path.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>