Allow playing in Record Attack with minor add-ons

Does what it says on the tin - complains only about events which could interfere with existing vanilla replays. Specifically:

• Replacing a map that already exists
• Any Lua script is added
• SOC events where:
  • A state or object that was not freeslotted in that file is modified
  • A level header is overwritten
  • A MainCFG block that does not create a custom gamedata is present

In addition, add a warning message when attempting to use the master server browser with add-ons loaded.

Basically, this allows most characters and levels currently available (notably including bonuschars.kart), both for earning emblems in existing levels and getting cool times in custom ones. I think you will all agree that this is a pleasing development.

It SEEMS scary with 18 files changed, sure, but it's not, I promise. Most of those are due to a change to G_SetGameModified's function signature to add a Boolean for "major-ness" of incident.