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.

.luacheckrc

@@ -20,5 +20,8 @@ read_globals = {
 -- Overwrites minetest.handle_node_drops
 files["mods/creative/init.lua"].globals = { "minetest" }
 
+-- Overwrites minetest.calculate_knockback
+files["mods/player_api/api.lua"].globals = { "minetest" }
+
 -- Don't report on legacy definitions of globals.
 files["mods/default/legacy.lua"].global = false