ctlHandler is passed to RegisterServiceCtrlHandlerEx as a callback
function. But all ctlHandler parameters have to be uintptr aligned
regardless of their type.

Adjust ctlHandler parameters, so they are aligned accordingly.

Also uncomment part of TestExample that verifies passing of
ChangeRequest.Context.

Fixes golang/go#25660

Change-Id: Ie96fa5c78b911ad9df6775f5c49e7f6d56464f6b
Reviewed-on: https://go-review.googlesource.com/c/158698Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>