runtime: set SIGCONT to _SigNotify + _SigDefault

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/18185
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/os/signal/signal_test.go b/src/os/signal/signal_test.go
index 7942e78..291c5f1 100644
--- a/src/os/signal/signal_test.go
+++ b/src/os/signal/signal_test.go
@@ -278,3 +278,12 @@
 		}
 	}
 }
+
+// Test that SIGCONT works (issue 8953).
+func TestSIGCONT(t *testing.T) {
+	c := make(chan os.Signal, 1)
+	Notify(c, syscall.SIGCONT)
+	defer Stop(c)
+	syscall.Kill(syscall.Getpid(), syscall.SIGCONT)
+	waitSig(t, c, syscall.SIGCONT)
+}