loop reaping dead children

Current code isn't very correct: because Linux signal system design
follows SysV Unix behavior, when a SIGCHLD happens, it only means there
are child programs died (either self-quit or involuntarily killed, from
wstatus may figure out), linux kernel only deliver SIGCHLD once to parent
before the parent process;
so when SIGCHLD happens, you don't know is that one child only or more

so the code need a loop until wait4 returns zero, at rumtime got this:

```console
2017/07/14 21:14:32 reap dead child: 579, wstatus: 0x00000000
2017/07/14 21:14:32 reap dead child: 580, wstatus: 0x00000000
2017/07/14 21:14:32 reap dead child: 583, wstatus: 0x00000000
```

then in the container there are no more zombies. final resolve #26

sigchld_unix.go

@@ -3,6 +3,7 @@
 package main
 
 import (
+	"log"
 	"os"
 	"os/signal"
 	"syscall"
@@ -23,6 +24,18 @@ func watchChildSignal() {
 }
 
 func reapChildren() {
-	var wstatus syscall.WaitStatus
-	syscall.Wait4(-1, &wstatus, syscall.WNOHANG, nil)
+	for {
+		var wstatus syscall.WaitStatus
+		wpid, err := syscall.Wait4(-1, &wstatus, syscall.WNOHANG, nil)
+		if err != nil {
+			log.Printf("syscall.Wait4 call failed: %v", err)
+			break
+		}
+
+		if wpid != 0 {
+			log.Printf("reap dead child: %d, wstatus: %#08x", wpid, wstatus)
+		} else {
+			break
+		}
+	}
 }