Respawn invulnerability refactor and fixes
I had these changes sitting on my USB drive for almost two years, so I thought I'd touch them up and create a merge request to push them in now. This refactors the respawn invulnerability code while keeping the original GZDoom code intact as much as possible to reduce delta, and addresses 0004135, 0004136, and 0004137 on the tracker.
- This adds the native
APowerRespawnInvulnerable
class and removes the now obsoleteSVC_PLAYERRESPAWNINVULNERABILITY
server command. - Lots of code simplifications.