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