kbdcontrol: enable pre-Unicode dead key table compatibility

The definition of pre-Unicode keymap ioctls will be made optional and
dependent on COMPAT_FREEBSD13 in a follow-up commit to 14-CURRENT.

While we generally provide ABI compatibility for older binaries on
a new kernel, but not functionally extended userland programs on an
old kernel, it has been specifically requested to preserve ABI
compatibility for the kbdcontrol program for both these cases.

Passing the kernel configuration option COMPAT_FREEBSD13 to the build
of kbdcontrol will make ioctls visible to the build that are normally
hidden, but required to implement compatibility with kernels that only
support 8 bit characters in dead key maps.

This commit is not to be merged to any previous FreeBSD version and
it shall be reverted as soon as this type of ABI compatibility is no
longer deemed necessary (probably before 14-STABLE is branched).

This commit is a part of review D38465 and split off to allow it to be
reverted using the commit ID.