Print default file names for command-line help.

Print default file names for command-line help.

Print the value set with defaultValueIfSet() for a file name option in
the command-line help if no other value is provided for the option.

- The value is formatted by the AbstractOptionStorage::formatValue()
  pure virtual method when a special index is provided.
- OptionStorageTemplate implements handling of this special index, and
  provides a new pure virtual method formatSingleValue() to do the
  actual formatting, which no longer needs to know where the value comes
  from.
- Adjust the concrete option storage classes to the changes.

IssueID #666.

Change-Id: I8b51262042415f314bd5d4c8da51e6e31cfe3b21