Enable mount.zfs helper

Enable mount.zfs helper

While not strictly required to mount a zfs filesystem using a
mount helper has certain advantages.

First, we need it if we want to honor the mount behavior as found
on Solaris.  As part of the mount we need to validate that the
dataset has the legacy mount property set if we are using 'mount'
instead of 'zfs mount'.

Secondly, by using a mount helper we can automatically load the
zpl kernel module.  This way you can just issue a 'mount' or
'zfs mount' and it will just work.

Finally, it gives us common hook in user space to add any zfs
specific mount options we might want.  At the moment we don't
have any but now the infrastructure is at least in place.