Treat strings returned from primitives as UTF-8 coded.

Treat strings returned from primitives as UTF-8 coded.

In particular, make sure that filenames are treated this way, because this is
normal for modern unix systems.  This change mostly affects filenames, but also
environment variables and command-line strings.

This is necessary because strings passed to primitives are converted to UTF-8
bytevectors.  Otherwise, a non-ASCII filename returned by a primitive won't be
converted back to the same bytes when passed to another primitive.