Use 'ps <pid>' to test for process existence if posix is not available

Summary: posix may not be loaded on the web/cgi SAPI but we call posix functions
on this pathway, which we hit on /daemon/. Fall back to exec if we don't have
posix.

Test Plan: Added "&& false" and verified the page executed a bunch of "ps"
tests.

Reviewers: Koolvin, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T821

Differential Revision: https://secure.phabricator.com/D1540