Expose task, and scheduler name information to debuggers

[mstewartgallus]

Currently, when debugging my heavily concurrent Rust program all I get is blanks in the name fields. It'd be really nice if my debugger could show the name I set using the task builder API.

This sort of relates to issue #2891 but is a different view on the topic.

CC @jdm

[thestinger]

Calling the pthread naming functions on every context switch seems like it would do this, but it might add too much overhead to what should be a cheap context switch and it's not portable.

[thestinger]

This will be very easy to implement now that there are only going to be native threads. (#17325)

[vojtechkral]

@thestinger I had a look as to how this could be implemented and it's actually not so easy, as there is no POSIX-ly standard way to set a thread's name. All the *nixes do this in a bit different way with the most annoying one being OS X where you only can set the name from within the thread and the function is only available since OS X 10.6. It seems Linux and BSDs are okay though.

Source: this SO thread.