std: Tweak rt::at_exit behavior

There have been some recent panics on the bots and this commit is an attempt to
appease them. Previously it was considered invalid to run `rt::at_exit` after
the handlers had already started running. Due to the multithreaded nature of
applications, however, it is not always possible to guarantee this. For example
[this program][ex] will show off the abort.

[ex]: https://gist.github.com/alexcrichton/56300b87af6fa554e52d

The semantics of the `rt::at_exit` function have been modified as such:

* It is now legal to call `rt::at_exit` at any time. The return value now
  indicates whether the closure was successfully registered or not. Callers must
  now decide what to do with this information.
* The `rt::at_exit` handlers will now be run for a fixed number of iterations.
  Common cases (such as the example shown) may end up registering a new handler
  while others are running perhaps once or twice, so this common condition is
  covered by re-running the handlers a fixed number of times, after which new
  registrations are forbidden.

Some usage of `rt::at_exit` was updated to handle these new semantics, but
deprecated or unstable libraries calling `rt::at_exit` were not updated.

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