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