Respect object property hp_max field for players (#6287)

* Respect object property hp_max field for players
This allows modders to configure the maximal HP per player

* Statbars: Downscale bar to full 20 HP when exceeding this value
Add default max HP for players and breath constants to builtin
Document the constants

* Rename PLAYER_MAX_HP -> PLAYER_MAX_HP_DEFAULT

doc/lua_api.txt

@@ -3276,6 +3276,7 @@ This is basically a reference to a C++ `ServerActiveObject`
         * `0`: player is drowning,
         * `1`-`10`: remaining number of bubbles
         * `11`: bubbles bar is not shown
+        * See constant: `minetest.PLAYER_MAX_BREATH`
 * `set_attribute(attribute, value)`:
     * Sets an extra attribute with value on player.
     * `value` must be a string.
@@ -3980,6 +3981,7 @@ Definition tables
 
     {
         hp_max = 1,
+    --  ^ For players, the maximal HP defaults to `minetest.PLAYER_MAX_HP_DEFAULT`
         physical = true,
         collide_with_objects = true, -- collide with other objects if physical = true
         weight = 5,