Improve interactive selection input

Improve interactive selection input

- Print the static index groups always at the start of the prompt.
- Add a hint for that an empty line provides information about the
  available index groups.
- Print the whole prompt again after an empty input line.
- Print only the selections from the current interactive prompt in the
  output.  This makes the listing clearer for commands like
    gmx sasa -surface -output
  when providing the latter set of selections interactively.
- Print the number of selections still required after the above list.
- Work better (on Unix) if stdin is from a pipe (using the same
  mechanism that was there in 4.5 and 4.6).

Change-Id: I38152fb4fccd142aeee318739ab3782fa6a775d8