Modify ktr(4) to be typesafe

Modify ktr(4) to be typesafe

- Use a struct copy to place the logged values directly into the
  destination buffer without forcing a memcpy.
- This also fixes event logging on x86_64, as we no longer rely
  on the i386 calling convention for functions with a variable
  number of arguments.
- Verify that the arguments match the specified format.
- Update callsites.

Note that this requires userspace changes! ktrdump(8) needs to be
adjusted even on i386 (%c and %hd are no longer promoted as per the
C rules). We need very special handling on x86_64 too.