Main function in LLVM should either take argv/argc arguments or not exist at all

Currently, the `main` function in generated LLVM programs takes no "normal" arguments and an environment structure pointer. However, the main function for executable programs should instead take the traditional C argc/argv arguments so it can be invoked. (As an additional optimization, we could even imagine disabling the environment pointer argument completely when a function has no free variables—and `main` is guaranteed to be closed.)

On the other hand, when we just want to link the generated code into a larger program, perhaps it would be nice to have an option to disable the main function altogether.