The Linux kernel adds a few evdev keycodes roughly every other release. These
aren't available as keysyms through XKB until they have been added as keycode
in xkeyboard-config and mapped there to a newly defined keysym in the X11
proto headers.

In the past, this was done manually,  a suitable keysym was picked at
random and the mapping updated accordingly. This doesn't scale very well and,
given we have a large reserved range for XF86 keysyms anyway, can be done
easier.

Let's reserve the range 0x10081XXX range for a 1:1 mapping of Linux kernel
codes. That's 4095 values, the kernel currently uses only 767 anyway. The
lower 3 bytes of keysyms within that range have to match the kernel value to
make them easy to add and search for. Nothing in X must care about the actual
keysym value anyway.

Since we expect this to be parsed by other scripts for automatic updating, the
format of those #defines is quite strict. Add a script to generate keycodes as
well...