include: let FindFileInXkbPath take a name+len instead of c-string

In the rules file parser we use pascal strings.

Signed-off-by: Ran Benita <ran234@gmail.com>
rules: add include statement

This allows a user to extend an existing rules file, or to break a large
file to more manageable pieces.

Needed for:
https://bugs.freedesktop.org/show_bug.cgi?id=74422

Signed-off-by: Ran Benita <ran234@gmail.com>
wip: context: add /etc/X11/xkb to default include path

Signed-off-by: Ran Benita <ran234@gmail.com>