Replaces magic numbers across the code base with proper constants.
Prepares for the ability to have more than one antag channel in the future.
Corrects a lie.
Expands on the earlier engine pump fix to add proper control over the
engine core atmos. Adds core control computer that can set output
external pressure bound instead of output pressure, and special high
power vent pump type for the engine core.
Docking ports still don't use filters though, as it would mess with the
docking ports that are also airlocks. Docking is infrequent though so it
should be fine.
