Percent‐encode filenames when generating u·r·i’s

It’s not known or expected whether tools like `xmlcatalog` can handle
full leiris. It’s better and safer to just only use u·r·i’s for
identifying resources.

Note that this _does_ have implications on includes (they must also be
percent‐encoded). Ideally, it would be possible to run this conversion
in the transforms, but this probably is not possible in X·S·L·T 1.0.