os: add ExitCode() method to ProcessState

Author: wgliang

src/os/exec_plan9.go

@@ -136,3 +136,7 @@ func (p *ProcessState) String() string {
 	}
 	return "exit status: " + p.status.Msg
 }
+
+func (p *ProcessState) ExitCode() int {
+	return p.status.ExitStatus()
+}

src/os/exec_posix.go

@@ -106,3 +106,7 @@ func (p *ProcessState) String() string {
 	}
 	return res
 }
+
+func (p *ProcessState) ExitCode() int {
+	return p.status.ExitStatus()
+}