Make rekillable consistent with unkillable

[flaper87]

As for now, rekillable is an unsafe function, instead, it should behave
just like unkillable by encapsulating unsafe code within an unsafe
block.

This patch does that and removes unsafe blocks that were encapsulating
rekillable calls throughout rust's libs.

Fixes #8232

[flaper87]

@bblum here it is 😃

[flaper87]

@bblum I added a small test but I'm not 100% that's what is needed. I'll add more if so. I'd appreciate if you could elaborate more on the various cases you think worth testing. (I'm pretty new to rust).

Thanks a lot!

[bblum]

The test you added looks good but there should also be a test for the advertised behaviour, namely, that a task can get killed inside a rekillable block. Something like this:

let result = do task::try {
    do unkillable { do rekillable {
        let (port,chan) = comm::stream();
        do task::spawn { chan.send(()); fail!(); }
        port.recv(); // wait for child to exist
        port.recv(); // block forever, expect to get killed.
    } }
};
assert!(result.is_err());

You also have not added any code to the runtime that will actually make the task fail, which you will need to do if you want the tests to pass.

[huonw]

This needs a rebase (it no longer merges cleanly).

[flaper87]

Will do!

I'm out and I'll be back tomorrow!!!

[flaper87]

rebased