Guarantee that native tasks wait for children

[alexcrichton]

There was a scheduling race where a child may not increment the global task
count before the parent exits, and the parent would then think that there are no
more tasks left.

Closes #11039

[huonw]

Is it worth having a test? e.g. a run-pass one like:

extern mod native;

static mut set: bool = false;
#[start]
fn start(argc: int, argv: **u8) -> int {
    native::start(argc, argv, proc() spawn(proc() unsafe {set = true}));

    if unsafe {set} {0} else {1}
}

[alexcrichton]

Excellent idea, I have added a test.