Perform some cleanups to devname(3).

Perform some cleanups to devname(3).

- Make sure the mode argument is either a character or a block device.
- Use S_IS*() instead of checking S_IF*-flags by hand.
- Don't use kern.devname when the argument is already NODEV.
- Always call snprintf with the proper amount of arguments corresponding
  with the format.
- Perform some whitespace fixes. Tabs instead of 4 spaces, missing space
  for return statement.
- Remove unneeded includes.