sys/kbio.h: support Unicode key codes in vt keymap files

Some keyboard definitions return Unicode characters that cannot be
represented in the 8 bits provided by an u_char.

Extend then width of the keycode entries to 16 bits to allow for all
keycodes currently defined in share/vt/keymaps/*,kbd.

Reported by: yuri@aetern.org
MFC after: 3 days