shutdown function should be marked unsafe

[DavidThien]

Version
>= 1.8.1

Platform
Linux system76-pc 6.2.6-76060206-generic #202303130630~1689015125~22.04~ab2190e~dev-Ubuntu SMP PREEMPT_DY x86_64 x86_64 x86_64 GNU/Linux (any)

Description
The shutdown function not properly marked unsafe.

I've been looking at some past bugs in tokio, and came across #3929 where abort changed from calling shutdown to remote_abort.

tokio/tokio/src/runtime/task/join.rs

Lines 193 to 197 in 8439494

	pub fn abort(&self) {
	if let Some(raw) = self.raw {
	raw.remote_abort();
	}
	}

This change was made because shutdown isn't safe to call in some contexts, or it might cause the original issue. It seems that the current shutdown function should be marked unsafe, because the code hasn't changed, and you can still call it in a way that causes the program to go wrong. Rust conventions seem to indicate this should be marked unsafe.

tokio/tokio/src/runtime/task/raw.rs

Lines 233 to 236 in a0a58d7

	pub(super) fn shutdown(self) {
	let vtable = self.header().vtable;
	unsafe { (vtable.shutdown)(self.ptr) }
	}

This isn't a safety issue currently, but could cause one in the future.