Add mutexes to all calls in std::rl

[brson]

This module is unsafe because the linenoise library is not threadsafe. Use core::private::global_data to store a global std::sync::Mutex and use that mutex to protect all the calls in rl.

Then make all the functions safe: #3921

[brson]

Also, like core::os::getenv, please leave a note in the comments indicating that this should use a truly global mutex once the language supports global variables (global_data is runtime-global).

[catamorphism]

I guess this really could break existing code, though it doesn't look like it was ever nominated. Commenting to reset the counter.

[huonw]

Triage; still an issue.

I can't see a global_data-like module anywhere any more, although this is probably unnecessary now that we have unsafe mut and can store a global mutex directly.

[brson]

Not sure why this is on a milestone. Removing.

[huonw]

Closed by #9096.