Modernize editor selector

Summary:
FreeBSD doesn't have nano by default, so allow falling back to vi. Also added sensible-editor.
This mostly retains the order we select things, but reduces some lines.

Test Plan: arc diff

Reviewers: epriestley, amckinley, kdhp, Blessed Reviewers

Reviewed By: epriestley, kdhp, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18141