URL: update "file:" URL handling

* LOTS of comment updates
* explicit error for SMB shares (e.g. "file:////share/path/file")
* more strict handling of authority (i.e. "//localhost/")
* now accepts dodgy old "C:|" drive letters
* more precise handling of drive letters in and out of Windows
  (especially recognising both "file:c:/" and "file:/c:/")

Closes #2110