Support Unicode characters in keymap dead key tables

Support for Unicode characters had been added to the keyboard code,
but there are keymaps that have accented characters accessed via dead
key combinations, and those were still restricted to 8 bit codes.

This update to kbd.c adds support for Unicode characters and
compatibility code that allows a kbdcontrol command built from kbio.h
without these patches to work on a new kernel.

Compatibility code that allows a new kbdcontrol binary running on an
old kernel to load and display the dead key map will be committed in a
separate commit.

Reviewed by: imp, brooks
Approved by: brooks
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D38381