- Semantics change of file_unpack.pl:
  Default to one_shot unless $0 contains 'deep'. New --deep option to also switch this one.
  This is a move towards least surprise. No other unpacker has this deep feature.

- Fixed builtin helper for application=x-lzip to conform to IPC::Run::run() semantics.
- Added an example how to write a mime helper script to the documentation.
- Fixed diagnostics in case a helper script fails.
- Several smallish doku improvements.
- Updated test suite to accept encoding 'unknown' as a synonym to 'binary'.