Bugfix: Fix kqueue event clobbering

[machty]

Description

Fixes root issue described here:

https://github.com/socketry/async/issues/137

The solution is to set the EV_UDATA_SPECIFIC flag on all kqueue events, which tells kqueue to consider the udata value (the pointer to the fiber) as part of the unique tuple.

Types of Changes

• Bug fix.

Testing

• [ ] I added tests for my changes.
• [x] I tested my changes locally.
• [ ] I tested my changes in staging.
• [ ] I tested my changes in production.

Test case output

Reprinting the simplified test case from https://github.com/socketry/async/issues/137:

Async do |t|
  puts "Root start"
  t.async do
    puts "Sleep 1 Start"
    sleep 1
    puts "Sleep 1 End"
  end
  t.async do
    puts "Sleep 2 Start"
    sleep 1
    puts "Sleep 2 End"
  end
  puts "Root end"
end

Without this PR, Ruby 3.1.0 prints the following output and hangs:

Root start
Sleep 1 StartSleep 2 Start
Root end
Sleep 2 End

With this PR on Ruby 3.1.0, the following is output (and doesn't hang):

Root start
Sleep 1 StartSleep 2 Start

Root end
Sleep 1 EndSleep 2 End

=> true

I tried this PR against your Ruby PR but the output is the same (it succeeds, doesn't hang).

[ioquatix]

Wow this is amazing, I didn't know this was possible!