Extension completion for FileNameOption.

Extension completion for FileNameOption.

- Make FileNameOption to complete extensions for input files, like the
  old file options did.
- Add descriptions for all file name options to improve the help output.
- Make defaultValueIfSet() really appear in the help output: it worked
  only in the case the option value was not stored.
- Adjust the reference for command-line help tests to match the new
  output.

Part of #642.

Change-Id: I4a749e3e82a42a6e64e0a78c91120f96a00381b9