[RFC] Remove shortname option and support for filesystems that limit the length of filenames to 8 characters

Justification: it's 2014.

This PR also includes a commit from #605. I suggested that @watk removed `shortname` support in that same PR as I think it makes more sense to remove the 'SHORT_FNAME` and `USE_LONG_FNAME` macros along with the `shortname` support.