xkb: use system defaults instead of "us" for layout

xkb: use system defaults instead of "us" for layout

libxkbcommon has a configure option to set the default values for the
RMLVO's (if they are passed as NULL or "" - we use "" so it'd be safe to
print). If they are not specified there, it's just "us" like we currently
do have. But if they are specified, we should most likely defer to it.

We do the same for the model field, instead of hardcoding "evdev". The
rules field remains hardcoded to "evdev", because we rely on it when
doing the evdev scancode-to-keycode +8 mapping.

Signed-off-by: Ran Benita <ran234@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>