chgserver: implement chgui._runpager This patch implements chgui._runpager in a relatively simple way. A more clean way is to move the core logic of "attachio" to "ui", which will be done later after chg runs uisetup per request.

chgserver: make S channel support pager request This patch adds the "pager" support for the S channel. The pager API allows running some subcommands, namely attachio, and waiting for the client to be properly synchronized.