Inappropriate `panic!`s in `Thread::new` on certain unix platforms

Found after discussing code surrounding #160219 on zulip.

Edit: It is inappropriate for Thread::new to ever panic, because we guarantee that thread::current, which uses it, does not call the global allocator reentrantly. The memory leak on unwind, which this issue was originally about, is not that important.

The implementation of Thread::new manually initializes a struct behind MaybeUninit, which means that if Parker::new_in_place unwinds, the name's memory (essentially Option<CString>), now held by the previously written name field behind MaybeUninit, is leaked.

https://github.com/rust-lang/rust/blob/4667d75565e47ba5df36c0df598c556b543e8624/library/std/src/thread/thread.rs#L101-L108

Note that this is possible, on certain unix platforms:

https://github.com/rust-lang/rust/blob/4667d75565e47ba5df36c0df598c556b543e8624/library/std/src/sys/sync/thread_parking/pthread.rs#L24-L32

which calls

https://github.com/rust-lang/rust/blob/4667d75565e47ba5df36c0df598c556b543e8624/library/std/src/sys/pal/unix/sync/condvar.rs#L152-L175

which uses assert_eq to ensure that the initialization of the Condvar succeeds (it can fail, e.g. from resource exhaustion). Note that the method itself expects failures are possible here. It uses a guard struct to ensure that the resources of the condattr are not leaked on unwind.

Furthermore, the implementation of std::thread::current seems to assume that Thread::new never unwinds, because it sets a flag to detect reentrance, which is never reset if an unwind occurs:

https://github.com/rust-lang/rust/blob/4667d75565e47ba5df36c0df598c556b543e8624/library/std/src/thread/current.rs#L289-L301

The first issue should be fixable by reordering the fields' initialization logic, and the second one by adding a drop guard that unsets the busy state.

Read library/std/src/thread/thread.rs around Thread::new, then trace Parker::new_in_place through library/std/src/sys/sync/thread_parking/pthread.rs and library/std/src/sys/pal/unix/sync/condvar.rs. Also inspect library/std/src/thread/current.rs around the reentrance flag; done means Thread::new does not panic in the described failure paths and current's state remains correct if unwinding occurs.