Use NFS for shared folders

The default Vagrant shared folders filesystem driver is not 100%
POSIX-compliant.  Things like hard links do not work at all.  Vagrant
can setup an NFS export automagically if host-only networking is
in use, so make use of it for greater compatibility.

Described in: https://docs.vagrantup.com/v2/synced-folders/nfs.html

Caveats: ignored completely in Windows, and requires superuser
access on the host system