NT-style paths that have a device prefix (\??\) also return the drive
of the current working directory (even if they're valid devices).

Paths that are not NT and not even close to DOS don't actually fail
catastrophically.  Even though MSDN suggests that it returns the boot
drive in this case, tests indicate that it returns the drive of the
current working directory.

Add support for "semi-DOS" paths, these paths revert to the drive
letter specified in the first character.

The purpose of this function is to return the most fundamental path
without leaving a filesystem.  Steam uses this so that it can use inode
searches, without this functionality some installations/validations will
fail if the Steam Library is not on the same drive as Steam
itself (symlink'd to another location).

This reverts commit 443ef6e7.
The tests fail on all versions of Windows.

Based on patch by Huw Davies.