Update mail-exec

[Neilpang]

No description provided.

[yizhongw]

Yes, this is a neater solution if you don't care about the output. But if you want the program still output to stdout and stderr, this may not work. For example, if you run mail-exec some-command > run.out, this redirecton is useless here. I think we should not disturb this kind of use. Thanks for the kind and patient help!