When MSIE sends a full file path (in "intranet mode"), it does not
escape backslashes: "C:\dev\go\foo.txt", not "C:\\dev\\go\\foo.txt".

No known MIME generators emit unnecessary backslash escapes
for simple token characters like numbers and letters.

If we see an unnecessary backslash escape, assume it is from MSIE
and intended as a literal backslash. This makes Go servers deal better
with MSIE without affecting the way they handle conforming MIME
generators.

Fixes #15664.

Change-Id: Ia3b03b978317d968dc11b2f6de1df913c6bcbfcc
Reviewed-on: https://go-review.googlesource.com/32175Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>