mfs: implement QUALIFY_FILENAME for printer

It should be implemented also for files!!!
Currently, instead, every function does the qualification on its own,
but for printer this doesn't work because, if initially left unqualified,
it will be prepended with current drive, so we'll have to qualify things
like C:\LPT5 later, which would be a pile of hacks.

Therefore, the entire mfs.c have to be rewritten to properly
support QUALIFY_FILENAME