kbdcontrol.c: make pre-Unicode compatibility conditional

Support for the full range of Unicode character codes has been added
to the main keymap back in 2009, with compatibility shims added in
2011 (to support an older kbdcontrol command on a new kernel during
an upgrade from FreeBSD-8 to FreeBSD-9).

Unicode support for accented characters that are reached via dead key
combinations has been added just recently, again with compatibility
shims to allow all combinations of old/new kernel and old/new
kbdcontrol command to load and display the keymaps including the dead
key table. (But full Unicode in the dead key table requires both a new
kernel and kbdcontrol command.)

This commit makes the compatibility shims depend on the respective
compatibility ioctls (OGIO_KEYMAP, OPIO_KEYMAP, OGIO_DEADKEYMAP, and
OPIO_DEADKEYMAP) being defined in sys/kbio.h. This is true for all of
them in 13-STABLE, none in 12-STABLE (as of now), and will become
optional due to a follow-up commit to sys/kbio.h in -CURRENT.

This commit is the only part of review D38465 that should be merged
back to 12-STABLE and 13-STABLE.

MFC after: 1 month