mime, time, internal/syscall/windows/registry: use new registry package to simplify...

mime, time, internal/syscall/windows/registry: use new registry package to simplify code

This CL copies golang.org/x/sys/windows/registry into
internal/syscall/windows/registry (minus KeyInfo.ModTime to prevent
dependency cycles). New registry package is used in mime and time
packages instead of calling Windows API directly.

Change-Id: I965a5a41d4739b3ba38e539a7b8d96d3223e3d56
Reviewed-on: https://go-review.googlesource.com/9271
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>