kbdreg.h: include opt_kbd.h

This is a kernel-only file, so it's safe to include opt_kbd.h. However,
add #ifdef _KERNEL guards to emphasize that. And also move the include
of opt_kbd.h in atkbdcreg.h to inside the kernel guards. Nothing outside
the kernel in tree uses the rest of that file, but I'm less comfortable
moving the #ifdef _KERNEL to the top of that file.

Sponsored by: Netflix