Since Stop was introduced, it would revert to the system default for the
signal, rather than to the default Go behavior.  Change it to revert to
the default Go behavior.

Change-Id: I345467ece0e49e31b2806d6fce2f1937b17905a6
Reviewed-on: https://go-review.googlesource.com/18229Reviewed-by: Russ Cox <rsc@golang.org>