Feature: Save override for level header

Kind of like how 2.2 works, but has a default option for compat w/ 2.1

Hub @ https://github.com/TehRealSalt/SRB2/tree/save-override

changed title from Save override for level header to Feature: Save override for level header

changed the description

Makes sense, and a very useful feature. P_CanSave logic looks consistent with the old behavior. +1

mentioned in issue #38 (closed)

merged

mentioned in commit bea76aa4