player_api: Prevent knockback when player is set as attached

This is not directly related to player models but fits well
as a convenience feature in player_api.

api.lua

@@ -96,6 +96,15 @@ end)
 local player_set_animation = player_api.set_animation
 local player_attached = player_api.player_attached
 
+-- Prevent knockback for attached players
+local old_calculate_knockback = minetest.calculate_knockback
+function minetest.calculate_knockback(player, ...)
+	if player_attached[player:get_player_name()] then
+		return 0
+	end
+	return old_calculate_knockback(player, ...)
+end
+
 -- Check each player and apply animations
 minetest.register_globalstep(function(dtime)
 	for _, player in pairs(minetest.get_connected_players()) do