Support SIGINT for graceful daemon shutdown in libphutil

Summary: Ref T5855. Makes SIGINT mean "graceful shutdown" instead of being a synonym of SIGTERM.

Test Plan: See next revision.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

Subscribers: epriestley

Maniphest Tasks: T5855

Differential Revision: https://secure.phabricator.com/D10227