Show some "arc" help pages through a configurable pager, like "less"
Fixes T5420. Some "arc help ..." is long and most similar commands send this kind of output through a pager.
Use a pager in at least some cases.
Test Plan: Ran "arc help land", got pager output. Ran "arc help land | cat", got raw output.
Maniphest Tasks: T5420
Differential Revision: https://secure.phabricator.com/D21327