- Make invalids variable local to do_conv such that it prints the number
  of invalid characters of the current file instead of an accumulated
  value.
- Make do_conv return an error when invalid characters have been found.
  Return EXIT_FAILURE from main if any file contained invalid characters.
  This matches the behaviour of GNU iconv.
- Mark usage with __dead2 attribute.
- Make the long_options array const.
Attach the CUSE library and kernel module to the default FreeBSD
builds. Bump the FreeBSD version number.