Disable umount.zfs helper

Disable umount.zfs helper

For the moment, the only advantage in registering a umount helper
would be to automatically unshare a zfs filesystem.  Since under
Linux this would be unexpected (but nice) behavior there is no
harm in disabling it.

This is desirable because the 'zfs unmount' path invokes the system
umount.  This is done to ensure correct mtab locking but has the
side effect that the umount.zfs helper would be called if it exists.
By default this helper calls back in to zfs to do the unmount on
Solaris which we don't want under Linux.

Once libmount is available and we have a safe way to correctly
lock and update the /etc/mtab file we can reconsider the need
for a umount helper.  Using libmount is the prefered solution.