NIOPosix on Darwin: inherit main thread QoS (#2944)

### Motivation:

On Darwin, QoS (quality of service) of threads plays an important role,
especially on Apple Silicon machines with P-cores and E-cores. If you
spawn raw threads (like NIOPosix) and use a mechanism that doesn't
support QoS propagation (like reading/writing to networks -- like
NIOPosix does), it's recommended to default to the main thread's QoS.

Otherwise you'll always be at the default QoS for "legacy" threads which
means bad latencies, especially on Apple Silicon machines.

In a follow-up PR #2943 we're adding better configurability for thread
configuration.

### Modifications:

Default to main thread QoS on Darwin.

### Result:

Better latencies for applications with higher QoS classes.

Sources/NIOPosix/ThreadPosix.swift

@@ -43,7 +43,12 @@ private func sysPthread_create(
     args: UnsafeMutableRawPointer?
 ) -> CInt {
     #if canImport(Darwin)
-    return pthread_create(handle, nil, destructor, args)
+    var attr: pthread_attr_t = .init()
+    pthread_attr_init(&attr)
+    pthread_attr_set_qos_class_np(&attr, qos_class_main(), 0)
+    let thread = pthread_create(handle, &attr, destructor, args)
+    pthread_attr_destroy(&attr)
+    return thread
     #else
     #if canImport(Musl)
     var handleLinux: OpaquePointer? = nil