Add a -main-is flag to rustc

[catamorphism]

Add a -main-is flag to specify the name of the main function if it's called something other than main. Then allow it to be set if only trans is getting called, so the FIXME in front::test::fold_mod can be resolved.

[pcwalton]

Why not an attribute?

[catamorphism]

That would be ok too!

[brson]

Should be an attribute.

[catamorphism]

Closing this as I think #4207 covers the same issue and has more discussion.