prune: mark renamed and protected backups differently

While it makes no difference for pruning itself, protected backups are
additionally protected against removal. Avoid the potential to confuse
the two. Also update the description for the API return value and add
an enum constraint.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>

test/prune_backups_test.pm

@@ -189,7 +189,7 @@ sub generate_expected {
 		'volid' => "$storeid:backup/vzdump-$vmid-renamed.tar.zst",
 		'type'  => 'unknown',
 		'ctime' => 1234,
-		'mark'  => 'protected',
+		'mark'  => 'renamed',
 		'vmid'  => $vmid,
 	    },
 	) if !defined($type);
@@ -375,7 +375,7 @@ my $tests = [
 	    {
 		'volid' => "$storeid:backup/vzdump-lxc-novmid.tar.gz",
 		'ctime' => 1234,
-		'mark'  => 'protected',
+		'mark'  => 'renamed',
 		'type'  => 'lxc',
 	    },
 	],