Fix addons with custom gamedata not loading

Fixes #473 (closed).

When testing an addon with custom gamedata via ZB or loading it via the command line,G_LoadGameData fails at the very first gamedata check, which is if ((modded && !savemoddata)). This merge request makes W_InitFile only set modifiedgame directly, instead of calling G_SetGameModified which also sets savemoddata to be false.

While I didn't encounter any issues with this change after several tests, I'm not 100% sure if this fix has any unwanted side effects, so please review and test this branch thoroughly.

srb2win_avoid-savemoddata.exe