Use the current ability to say that we don't do anything with SIGCONT by
default, but programs can catch it using signal.Notify if they want.

Fixes #8953.

Change-Id: I67d40ce36a029cbc58a235cbe957335f4a58e1c5
Reviewed-on: https://go-review.googlesource.com/18185Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>