sys/kbio.h: make pre-unicode keymap support optional

FreeBSD-9 had introduced support for the full set of Unicode
characters to the parsing and processing of keymap character tables.

This support has been extended to cover the table for accented
characters that are reached via dead key combinations in FreeBSD-13.2.

New ioctls have been introduced to support both the pre-Unicode and
the Unicode formats and keyboard drivers have been extended to support
those ioctls.

This commit makes the ABI compatibility functions in the kernel
optional and dependent on COMPAT_FREEBSD13 in -CURRENT.

The kbdcontrol command in -CURRENT and 13-STABLE (before 13.2) has
been made ABI compatible with old kernels to allow a new world to be
run on an old kernel (that does not have full Unicode support for
keymaps).

This commit is not to merged back to 12-STABLE or 13-STABLE. It is
part of review D38465, which has been split into 3 separate commits
due to different MFC and life-time requirements of either commit.

Approved by: imp
Differential Revision: https://reviews.freebsd.org/D38465