fixes #6005 and broken ancient human low/high pressure code.

-fixes low/high pressure not damaging unsealed/damaged spacesuits.
-refactors procs and adds more comments.
-removes unused/dead code.
HAZARD_HIGH_PRESSURE and the like are deprecated. See if you can use species.hazard_high_pressure instead.

Actually, it would probably be best if you move those defines out of setup.dm and into species.dm, so that future people don't get confused as well.