Provide xutftowcs helper to canonicalize paths

This provides an xutftowcs helper, xutftowcs_canonical_path(),
since extremely long relative paths will cause errors with Windows
due to path length limitations.

Signed-off-by: Doug Kelly <dougk.ff7@gmail.com>
Merge pull request #86 from vangdfang/master

Canonicalize paths into xutftowcs_path