Only SIGINT the daemon itself, not its entire process group

Summary: Signaling the process group is correct for SIGTERM and SIGKILL (nongraceful exits) but not for SIGINT (graceful exit). Subprocesses of the daemon may exit abruptly on SIGINT, which is not consistent with the requested behavior ("exit gracefully").

Test Plan: Ran bin/phd start, bin/phd stop.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11889