Support BYSETPOS for RRULE events with FREQ=MINUTELY

Summary: Ref T10747. (Most of the MINUTELY tests in the source I'm cribbing from aren't too interesting so I omitted a chunk of them.)

Test Plan: Unit tests now pass.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16630