kbfsmount on Windows doesn't lock exclusive use of drive letter.

This issue appears to have been in Keybase for Windows for a while, and may relate (in part) to issue #2708.

On my system the mountpoint was changed to M: due to a conflict with K: (a pre-existing flash card reader). I discovered later I could still use the M: drive letter to assign to a new virtual disk or change an existing partitions drive letter assignment.
I used 'kbfsmount set' to change the letter to Q:, rebooted and verified the encrypted filesystem was now on Q:. However, I could still see Q: as available for assignment in the Windows 7 Disk Management tool, when selecting a hard drive partition and changing it's drive letter assignment.

Other drive letters in use by both physical hardware and virtual devices (for example a RAM drive) do not list as being available for use. I tested and verified it was possible to assign the kbfsmount drive letter to another device in Windows 7.

The expected behavior would be for Keybase to use the drive letter and then mark it as in-use in the OS via the appropriate API. It's not using that method as it shouldn't be able to mount over already assigned disks in the first place.

Start with the Windows implementation of kbfsmount and the entry point used by `kbfsmount set`; no source file or test is named in the issue. Reproduce the drive-letter availability check in Windows 7 Disk Management, then verify that a letter assigned to the encrypted filesystem is no longer offered for another device.

Written by the indexing model from the issue text.