vagrant: Add NFS backend for file synchronization for OSX.
vagrant: Add NFS backend for file synchronization for OSX.

This is intended to fix serious issues with frequent guest crashes on
OSX High Sierra with Virtualbox and the default file synchronization.

Note from Tim: We've tried this idea before, and it worked for Mac,
but caused problems for non-Mac users of our Vagrant setup that
resulted in it being reverted in
1432e9afb0acd0472e89e1af77404bd6b73097b6.

Hopefully the new conditional will handle things correctly in both Mac
and Linux.

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