Workaround for mounting ZFS file systems

1) continue added in _rootdev_detect after ZFS detected successfully
Otherwise, it will fall through to the block file check and fail.
2) Add ZFSROOT variable which is set to 1 on successful detection of ZFS file system in _rootdev_detect
blkid check will return nothing on a ZFS file system, so I set fstype to zfs_member in _rootdev_mount if ZFSROOT is 1