string-util: move version_is_valid() into generic code

string-util: move version_is_valid() into generic code

While we are at it, replace the sloppy use of filename_is_valid() by the
less sloppy filename_part_is_valid() (as added by the preceeding
commit), since we don#t want to be too restrictive here. (After all,
version strings invalid as standalone filenames might be valid as part
of filenames, and hence we should allow them).