Support an "--active" flag for selecting active tasks

Summary: Ref T13591. This is mostly a workaround for Big Sur not having pcntl/posix installed by default and the mess with M1 / Homebrew / SIP / Code Signing (see T13232) so I can't easily run actual daemons and need to fake them with bin/worker execute --active, but it's a reasonable flag on its own.

Test Plan:

• Ran bin/worker execute --active and bin/worker cancel --active.

Maniphest Tasks: T13591

Differential Revision: https://secure.phabricator.com/D21517