API to terminate hg serve process

[TK Soh]

In TortoiseHg's serve dialog, we have to brew our own 'service' class,
based on 'service' class in mercurial.commands.serve(), so that we can
add a stop() method for terminating the server from within the dialog.

I wonder if these requests would make sense:

1. move the service class from mercurial.commands.serve() to, perhaps,
mercurial.hgweb.common, so that we don't need to continue to track
changes in mercurial on this aspect;

2. add a 'stop' method to service class for terminating terminate the server.

IMHO, this should also help make the API more consistent in long run.

TK