arc set-config: warn about unknown keys
Summary: See T12266. Warn when the user tries to set a value we will probably not read.
Test Plan: arc set-config foo bar, see fancy warning.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D17357