Rewrite encoding handling.

This patch rewrites fixu8() and affects a couple of places where decode() was
used directly.

It has the benefit of removing the ugly previous function, replacing it
with... well, another ugly function, but with a different kind of uglyness.

It also supports multiple encodings, so if the first one fails, a second one
is tried (and so on).

There still are some corner cases with file named in alternative encodings,
but should work much better than the older code.