The existing implementation requires QUERY_VALUE and ENUMERATE_SUB_KEYS permissions to enumerate
subkeys, so, using registry key name limits, Stat function could be excluded
from methods body and improved method requires only ENUMERATE_SUB_KEYS permission

Registry elements size limits described there:
https://msdn.microsoft.com/library/windows/desktop/ms724872.aspx

Fixes golang/go#23869

Change-Id: Id96beb9b0b294f01cc6eb1bb53bee5f50d02ea7e
Reviewed-on: https://go-review.googlesource.com/95655Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>