fix(self-update): wait on inherited parent handles on Windows

[Cloud0310]

Motivation

This PR fixes the PID reuse race described here:

rustup/src/cli/self_update/windows.rs

Lines 441 to 444 in 4d24845

	// FIXME: Using the process ID exposes a race condition
	// wherein the parent process already exited and the OS
	// reassigned its ID.
	let parent_id = entry.th32ParentProcessID;

Method

• Add spawn_with_parent_handle to launch a subprocess with an inherited handle to its parent process.
• Pass the handle value through the RUSTUP_PARENT_HANDLE environment variable so the subprocess can wait for its parent to exit.

Compatibility

Older rustup versions do not pass a parent process handle value when launching the self-updater.
So the original wait_for_parent implementation is renamed as wait_for_parent_legacy and used when RUSTUP_PARENT_HANDLE is absent.

Tests

Add self_replace_waits_for_legacy_parent to simulate an older launcher starting the new updater without passing a parent handle, covering compatibility with older rustup versions that use the old parent PID protocol.

[Cloud0310]

@ChrisDenton I am thinking your previous suggestion upon using stdin for passing handles in #5088 (review).
If you think using a little bit of unsafe code as this new PR does in new spawn_with_handle function, I guess I would love to switch over to the new func.

[rustbot]

☔ The latest upstream changes (possibly #5106) made this pull request unmergeable. Please resolve the merge conflicts.