kmscon: add --xkb-keymap option for predefined keymaps

kmscon: add --xkb-keymap option for predefined keymaps

This adds an --xkb-keymap <FILE> option to kmscon. When given, kmscon
will try to compile the keymap from the file before trying the other
options (like the XkbKeymap option in xorg.conf).

This is useful for users who have a customized XKB keymap, which is
usually kept in a single file. Example, in X:
	xkbcomp $DISPLAY my_keymap.xkb
Customize my_keymap.xkb to your liking, and then in .xinitrc, or
xorg.conf:
	xkbcomp my_keymap.xkb $DISPLAY
Now you can also do this in kmscon.conf.

Additionally, lacking such an option, kmscon is quite difficult to use
without an installed xkeyboard-config package, which provides the
infrastructure for the "rules" configuration mechanism. We might even
want to distribute some plain xkb file as a last ditch, for
robustness, if even the default RMLVO fails... without a keyboard a
terminal is not very useful.

(changed Ran's patch to use *_from_string() instead of *_from_file())
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>