Fix string8 to handle UTF-8 filenames on Linux

[Imarok]

We should fix string8 not displaying UTF-8 encoded OsPaths on Linux. Also see this todo at ​https://github.com/0ad/0ad/blob/master/source/lib/path.h#L138:

// TODO: On Unixes, this will only be correct for ASCII or ISO-8859-1
		// encoded paths; we should probably assume UTF-8 encoding by default
		// (but take care to handle non-valid-UTF-8 paths safely).