Calling `hb.change_bar' with hunger bar on damage disabled server
will crash the server. To not overload other functions to check
`enable_damage' over and over, simply make `hb.change_bar' able to be
called with uninitialized identifiers. #4586
* added a way to have a damage modifier relying on type instead of flag
* added Slow Falling
* added Levitation
* added Health Boost
* added Absorption
Several mods set or unset the visibility of a HUD bar way too often (e.g.
in a globalstep handler), causing the server to send a lot of superfluous
HUDCHANGE packets to each client. Returning from hb.hide_hudbar() early
if HUD bar visibility would not change prevents sending these packets.