http: support sending hgargs via POST body instead of in GET or headers
narrowhg (for its narrow spec) and remotefilelog (for its large batch
requests) would like to be able to make requests with argument sets so
absurdly large that they blow out total request size limit on some
http servers. As a workaround, support stuffing args at the start
of the POST body.
We will probably want to leave this behavior off by default in servers
forever, because it makes the old "POSTs are only for writes"
assumption wrong, which might break some of the simpler authentication
configurations.