eden: add config and awareness for our edenfs.kext
eden: add config and awareness for our edenfs.kext

Summary:
This commit adds a new eden configuration option that
controls whether we try to load our edenfs.kext in preference to
an alternative fuse implementation on macOS.

The majority of this diff is plumbing to convey the configuration
value through to the privhelper, which is relatively restrictive
due to its root-ness.

I've also updated watchman and mercurial to be aware of the new
filesystem type that shows up in the mount table.

Reviewed By: genevievehelsel

Differential Revision: D25065462

fbshipit-source-id: 4f35b9440654298e2706a0d0613d97eb63451999

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