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.
These are not necessarily on the same volume, especially on Linux where you can mount a subdirectory to a different partition (you can do the same thing on Windows, but it's probably less likely there).

Although in this case it's unlikely that a user mounts a single book on a different partition.