Rework / fix numerous bugs in rust_kernel::fail

[graydon]

The last person in there was not too happy with it.

[bblum]

Running this enough times will eventually hang, as one task "escapes":

fn foo() {
    task::yield();
    task::spawn(foo);
}

fn main() {
    let b = task::builder();
    task::unsupervise(b);
    task::run(b, foo);
    for iter::repeat(1000) { task::yield(); }
    fail;
}