Fallback from $EDITOR to editor to configured fallback editor.

Summary: Debian, etc., provide an editor binary which is a better fallback
than nano if $EDITOR is not set. See T833.

Test Plan: Used test/interactive_editor.php to test a bunch of configurations.
Verified that "editor" is selected if available.

Reviewers: btrahan

Reviewed By: btrahan

CC: jtolds, aran, epriestley

Maniphest Tasks: T833

Differential Revision: https://secure.phabricator.com/D1566