Add phd reload to send SIGHUP to overseers

Summary: Ref T7384. This just sends SIGHUP to specified overseers in a nice package.

Test Plan: See D11898.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T7384

Differential Revision: https://secure.phabricator.com/D11899