Add a phutil_ini_decode function

Summary: Ref T5105. This function behaves similarly to phutil_json_decode but for INI strings.

Test Plan: Wrote unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T5105

Differential Revision: https://secure.phabricator.com/D11443