* groffer.sh: enhance version information

* groffer.sh: enhance version information
`groffer -v|--version' now print:
- groffer's version number and date
- the string `is part of '
- groff's version information (version number and copyright),
  but not groff's `called subprograms' information.

* groffer.sh: minor fixes
- Fix the argument parser to process argument `-' correctly.
- Some display programs have trouble with empty input; feed a
  line consisting of a single space character in this case for
  all display modes (not for source or groff modes).

* TODO:
fix entry for `shoop'.