mkintifs: nlplug-findfs mount zfs filesystems on import

Hello. As I see nlplug-findgs calls zpool import without any additional parameters during its work:
https://github.com/alpinelinux/mkinitfs/blob/master/nlplug-findfs.c\#L508

In that case imported pool’s filesystems are mounted by default if they have mounpoint properties.
When later initramfs pivots into new root, they are left there mounted and can not be used because zfs thinks they are already mounted and cannot unmount them at the same time.

I think it would be better to do zpool import with “-N” option to not mount filesystems by default and then mount specified root filesystem somehow.

Also, I think providing some means to specify “-f” option would be nice too in cases when pool that was imported to other system needs to be imported.

(from redmine: issue id 8609, created on 2018-03-05)