Peaceful mode and disabling achievements

[Zaflis]

As far as i understand, currently if you enable peaceful mode, some achievements are disabled. However if you use worldgen settings that makes no biters at all, you do get all achievements. There's not much difference, except that with peaceful it's even harder to play and so i don't see the reason to have that limitation anymore. It's propably easier to understand a rule where only /c commands disable achievements.