Enable allow_emerge by default in singleplayer

Currently `jumpdrive.allow_emerge` is disabled by default to prevent excessive uncontrolled mapgen on public multiplayer servers.
That's better for most multiplayer servers as there's no rate limiters included.

But I think it could be good to have that setting enabled by default for single player mode.

What do you think, would it be better or worse to have dynamic default state for this setting?