Change mount_nfs_folder cap to be more like the linux version, adding a
bunch of retires rather than a long sleep.

Add DarwinNFSMountFailed error. This might move be more appropriate at
the plugin level.

Integrate some of tvsutton's work in configure_networks to get the
implementation closer to working in both fusion and virtualbox.

Add shell_expand_guest_path capability (also copied from linux)