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.
This doesn't work on Linux. See my comment on the Palaso PR. 

Better use `return new Uri(drivePath).AbsoluteUri;`