D3649: serve: add option print-url

[Yuya Nishihara]

(resend because I forgot to remove the Phab shit, sorry for the noise.)

>   > It's surprising that `hg serve --print-url` spawns a daemon process!
>   
>   If it does not launch a daemon process, the procress caller (`xdg-open`, `midori`, `firefox`, etc.) waits the end of the stream to launch, so it waits that the `hg serve` procress ends!

You can just run `hg serve -d --print-url`.

The option could be named as `--daemon-print-url` if we needed such big
red switch, but I don't think that makes sense.