proc: bugfix: Detach(true) does not kill tracee

Instead of using PTRACE_DETACH to inject SIGINT into the tracee use
sys.Kill directly: PTRACE_DETACH is allowed to ignore its signal
argument if the tracee isn't in signal-delivery-stop status.

proc/proc.go

@@ -101,11 +101,13 @@ func (dbp *Process) Detach(kill bool) (err error) {
 		}
 	}
 	dbp.execPtraceFunc(func() {
-		var sig int
-		if kill {
-			sig = int(sys.SIGINT)
+		err = PtraceDetach(dbp.Pid, 0)
+		if err != nil {
+			return
+		}
+		if kill {
+			err = sys.Kill(dbp.Pid, sys.SIGINT)
 		}
-		err = PtraceDetach(dbp.Pid, sig)
 	})
 	return
 }