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.
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https://www.mercurial-scm.org/wiki/ContributingChanges
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273ce12ad8f1` will report some style violations on a very old commit).