[Config] Mount destinations MUST NOT be nested in Windows

[RobDolinMS]

This was raised during reviews with folks working on Windows Containers.

This squashes commits from PR #433

Signed-off-by: Rob Dolin RobDolin@microsoft.com

[crosbymichael]

LGTM

[wking]

bef035c looks good to me.

[mrunalp]

LGTM

[wking]

I was thinking about this a bit more, and have a few questions:

1. At what level are nested mounts not supported?
   a. Is it a Windows-kernel restriction? If so:

   i. Is the restriction because of a policy (e.g. “allowing nested
   mounts is an unacceptable foot-gun and will not be
   supported”), or
   ii. Is it because the kernel devs just haven't gotten around to
   adding support for it yet?

   b. If it is not a kernel restriction, is it a runtime restriction?
   If so, the same policy questions apply, as well as:

   i. Is the platform sufficiently open to support third-party
   runtimes (in which case those runtimes might support nested
   paths).

2. What counts as a nested path? The order is significant 1, and
   I'm curious about the following cases:

   i. Mounting into a subdirectory (e.g. c:\foo followed by
   c:\foo\bar).
   ii. Mounting exactly over (e.g. c:\foo followed by c:\foo).
   iii. Mointing over a parent directory (e.g. c:\foo\bar followed by
   c:\foo).

Under a few of the (1) cases, I'd suggest relaxing this PR's MUST NOT
to a MAY NOT.

And I'm guessing that at least (2i) is illegal, but I'm not sure about
the others.