Add xkb support

[legionus]

It's very nice to be able to use the keymap from the xkbcommon project. Using xkbcommon a keymap is created and then converted to a kernel keymap as far as possible.

This support does not replace regular keymaps. There are no plans for this. The command looks like:

loadkeys --xkb-layout="us,ru,fr" --xkb-options="grp:caps_toggle"

This implementation is inspired by the console-setup project. Many thanks to Anton Zinoviev for help.

At the moment compose is not implemented. This is because xkbcommon does not provide an API for iterating over a compose table. I hope to solve this problem in the future.