Fix buggy disable_sigint implementation

[davidanthoff]

From a Slack conversation with @vtjnash it seems that the current implementation of disable_sigint is buggy. I quote him here:

IIUC, defer_signal is inherited from the Task that switched to it, which is a terrible design and not one that we should keep, but is currently what it appears to implement

#49541 (comment) suggests that the intent was (or should be) that the defer_signal state should really be task-local.

Maybe making it properly task-local could count as a simple bug fix that then could also be backported to the LTS version?

I am trying to fix the Ctrl+C handling in the VS Code extension and I think I won't be able to make much progress unless this is fixed.

I believe the buggy code is essentially this

julia/src/task.c

Lines 710 to 740 in 36472a7

	// Store old values on the stack and reset
	sig_atomic_t defer_signal = ptls->defer_signal;
	int finalizers_inhibited = ptls->finalizers_inhibited;
	ptls->finalizers_inhibited = 0;
	jl_timing_block_t *blk = jl_timing_block_task_exit(ct, ptls);
	ctx_switch(ct);
	#ifdef MIGRATE_TASKS
	ptls = ct->ptls;
	t = ptls->previous_task;
	ptls->previous_task = NULL;
	assert(t != ct);
	assert(jl_atomic_load_relaxed(&t->tid) == ptls->tid);
	if (!t->sticky && !t->ctx.copy_stack)
	jl_atomic_store_release(&t->tid, -1);
	#else
	assert(ptls == ct->ptls);
	#endif
	// Pop old values back off the stack
	assert(ct == jl_current_task &&
	0 != ct->ptls &&
	0 == ptls->finalizers_inhibited);
	ptls->finalizers_inhibited = finalizers_inhibited;
	jl_timing_block_task_enter(ct, ptls, blk); (void)blk;
	fesetenv(&ct->fenv);
	sig_atomic_t other_defer_signal = ptls->defer_signal;
	ptls->defer_signal = defer_signal;

.

I know that there have been a lot of (wide ranging) discussions how SIGINT handling in general could be improved, but in my ideal world those would not hold up a simple bug fix here :)