Fix BL-734 & BL-712, allow windows UNC paths

There may be more path-related issues that will turn up with testing,
but this gets us past the obvious creation/editing/saving of a book
on //localhost, so that testing can commence.

This can be merged now, but half of the fix is in libpalaso, PR 131.
That gives you something like "file:///C:", not what we want either. I can have separate versions of this for windows & mono, but I don't know how the resulting path should look on Linux. Thanks for your help.