server: move cmdutil.service() to new module (API)
And call it runservice() because I'll soon add createservice().
The main reason I'm going to introduce the 'server' module is to solve
future dependency cycle between chgserver.py and commandserver.py.
The 'server' module sits at the same layer as the cmdutil. I believe it's
generally good to get rid of things from the big cmdutil module.
[hgk]
path = /usr/share/mercurial/hgk