Back out "launchd: launch Watchman from /bin/sh"
Back out "launchd: launch Watchman from /bin/sh"

Summary:
The original change broke Watchman for open source users. In the short term, let's replace this with a config knob that we can enable on Sandcastle and disable everywhere else.

Original commit changeset: 9c6f429de7a3

Original Phabricator Diff: D36386248 (https://github.com/facebook/watchman/commit/db90c1bfc87c76d5169ed4958b778efa2e82c188)

Reviewed By: xavierd

Differential Revision: D37115292

fbshipit-source-id: 533721ca3cf567a899bfba69619f9ef6b38e5ffd

facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman