os/signal: add Stop, be careful about SIGHUP

Fixes #4268.
Fixes #4491.

R=golang-dev, nightlyone, fullung, r
CC=golang-dev
https://golang.org/cl/7546048

src/pkg/os/signal/signal_unix.go

@@ -12,6 +12,7 @@ import (
 )
 
 // In assembly.
+func signal_disable(uint32)
 func signal_enable(uint32)
 func signal_recv() uint32
 
@@ -26,13 +27,27 @@ func init() {
 	go loop()
 }
 
-func enableSignal(sig os.Signal) {
+const (
+	numSig = 65 // max across all systems
+)
+
+func signum(sig os.Signal) int {
 	switch sig := sig.(type) {
-	case nil:
-		signal_enable(^uint32(0))
 	case syscall.Signal:
-		signal_enable(uint32(sig))
+		i := int(sig)
+		if i < 0 || i >= numSig {
+			return -1
+		}
+		return i
 	default:
-		// Can ignore: this signal (whatever it is) will never come in.
+		return -1
 	}
 }
+
+func enableSignal(sig int) {
+	signal_enable(uint32(sig))
+}
+
+func disableSignal(sig int) {
+	signal_disable(uint32(sig))
+}