usr.sbin/kbdcontrol.c: Add backwards compatibility functions

This commit allows a kbdcontrol binary built with a version of kbio.h
that supports Unicode characters in dead key maps to load and display
keymaps including the dead key tables on a kernel built with a
previous version of kbio.h (that only supported 8 bit characters in
the dead key map).

This commit is meant as a temporary compatibility shim that will be
reverted when it can be assumed that all relevant systems have been
upgraded to a kernel that uses the updated kbio.h.

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D38388

(cherry picked from commit b92f8e5c0dccd52ea194cabe835b7af0b5e91f09)