gh-126692 took care of one situation where we were assuming the thread ID did not change across fork. However, there is at least one other, in PyOS_AfterFork_Child(). This PR addresses that case along with making the parent's thread ID available to the child.

Here's a breakdown:

• add _PyRuntimeState.os_fork.mutex to allow only one instance of forking to happen at once (in part, to protect the rest of _PyRuntimeState.os_fork)
• add _PyRuntimeState.os_fork.parent.tid to make the parent's thread ID available to the child
• add _PyRecursiveMutex_at_fork_reinit(), which makes use of the parent thread ID
• use _PyRecursiveMutex_at_fork_reinit() in _PyImport_ReInitLock()

I've also moved the _PyImport_ReInitLock() call from PyOS_AfterFork_Child() to _PyRuntimeState_ReInitThreads(), to be more consistent about where we reinitialize locks after forking. If there are any objections, I don't mind dropping that part.

One motivation I have here is that I'd like to use a _PyRecursiveMutex elsewhere and need to be able to correctly reinitialize after forking. That requires knowing the parent's thread ID, to decide if the forking thread held the lock or not. (This wasn't a problem for the import lock, which is a _PyRecursiveMutex, since we always acquire and hold that lock while forking.)

@gpshead, I'd be particularly interested in your thoughts on this.

• Issue: CPython should not assume that pthread_self returns the same value in fork parent and child #126688