readspline: handle local file:// URLs

PCManFM passes file:// URIs as parameters when opening directly from the explorer. Decode the URL and treat it as a local file.

[Diff without whitespace](https://github.com/fontforge/fontforge/pull/1494/files?w=1)

Closes #1452.

r? @frank-trampe
What keeps this from being reset at line 986 or line 990?