path/filepath: fix match of \\?\c:\* on Windows

path/filepath: fix match of \\?\c:\* on Windows

\\?\c:\ is a "root directory" that is not subject to further matching,
but the ? makes it look like a pattern, which was causing an
infinite recursion. Make sure the code understands the ? is not a pattern.

Fixes #15879.

Change-Id: I3a4310bbc398bcae764b9f8859c875317345e757
Reviewed-on: https://go-review.googlesource.com/31460
Reviewed-by: Quentin Smith <quentin@golang.org>