pager: do not special case chg Since chg has its own _runpager implementation, it's no longer necessary to special-case chg in the pager extension. This will effectively enable the new chg pager code path that runs inside runcommand.

chg: remove getpager support We have enough bits to switch to the new chg pager code path in runcommand. So just remove the legacy getpager support. This is a red-only patch, and will break chg's pager support temporarily.

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.

chg: handle pager request client-side This patch implements the simple S-channel pager handling at chg client-side. Note: It does not deal with environ and cwd currently for simplicity, which will be fixed later.

chgserver: use util.shellenviron This avoids code duplication.