Where available, use async_signals in libphutil

Summary: Ref T9640. In libphutil, switches to the PHP 7.1 async_signals mechanism when it is available.

Test Plan:
With the next change, ran daemons with bin/phd start and bin/phd debug, then ^C'd them.

• Before patch: long hang.
• After patch: They responded to the signals normally and exited promptly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9640

Differential Revision: https://secure.phabricator.com/D17200