Create zap for root vdev

And add it to the AVZ, this is not backwards compatible with older pools
due to an assertion in spa_sync() that verifies the number of ZAPs of
all vdevs matches the number of ZAPs in the AVZ.

Granted, the assertion only applies to #DEBUG builds - still, a feature
flag is introduced to avoid the assertion, com.klarasystems:vdev_zaps_v2

Notably, this allows to get/set properties on the root vdev:

    % zpool set user:prop=value <pool> root-0

Before this commit, it was already possible to get/set properties on
top-level vdevs with the syntax <type>-<vdev_id> (e.g. mirror-0):

    % zpool set user:prop=value <pool> mirror-0

This syntax also applies to the root vdev as it is is of type 'root'
with a vdev_id of 0, root-0. The keyword 'root' as an alias for
'root-0'.

The following tests have been added:

    - zpool get all properties from root vdev
    - zpool set a property on root vdev
    - verify root vdev ZAP is created

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Wing <rob.wing@klarasystems.com>
Sponsored-by: Seagate Technology
Submitted-by: Klara, Inc.
Closes #14405

lib/libzfs/libzfs_pool.c

@@ -2859,6 +2859,7 @@ zpool_vdev_is_interior(const char *name)
 	    strncmp(name, VDEV_TYPE_SPARE, strlen(VDEV_TYPE_SPARE)) == 0 ||
 	    strncmp(name,
 	    VDEV_TYPE_REPLACING, strlen(VDEV_TYPE_REPLACING)) == 0 ||
+	    strncmp(name, VDEV_TYPE_ROOT, strlen(VDEV_TYPE_ROOT)) == 0 ||
 	    strncmp(name, VDEV_TYPE_MIRROR, strlen(VDEV_TYPE_MIRROR)) == 0)
 		return (B_TRUE);