Always implicitly add 'return 0' to extern(C) main functions

[ibuclaw]

I'm not sure why we need -betterC in order to do this.

It is possible to have:

module object;
extern(D) void main() { }
extern(C) void main() { }

And it's expected that the program should return 0.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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