More elaborate default handling for FileNameOption
- FileNameOption now prefers existing files when completing the default
basename to a full filename for input options, like t_filenm parsing
does.
- Add FileNameOptionManager that allows interaction between different
file-related options. Use it to add support for -deffnm-like option
to set a default base name for all FileNameOptions.
To do the above, and make the -deffnm option independent from the
command line order, the default file names are now handled during
Options::finish(). So there are cases when the value returned by a file
name option changes after this call.
Change-Id: I73d3badae5409e9f066f06dec307b2c10ac89bfd