Avoid using stderr explicitly

Sources/NIOPosix/SelectableEventLoop.swift

@@ -19,6 +19,22 @@ import NIOConcurrencyHelpers
 import NIOCore
 import _NIODataStructures
 
+private func printError(_ string: StaticString) {
+    string.withUTF8Buffer { buf in
+        var buf = buf
+        while buf.count > 0 {
+            // 2 is stderr
+            let rc = write(2, buf.baseAddress, buf.count)
+            if rc < 0 {
+                let err = errno
+                if err == EINTR { continue }
+                fatalError("Unexpected error writing: \(err)")
+            }
+            buf = .init(rebasing: buf.dropFirst(Int(rc)))
+        }
+    }
+}
+
 /// Execute the given closure and ensure we release all auto pools if needed.
 @inlinable
 internal func withAutoReleasePool<T>(_ execute: () throws -> T) rethrows -> T {
@@ -438,12 +454,11 @@ internal final class SelectableEventLoop: EventLoop, @unchecked Sendable {
                     if Self.strictModeEnabled {
                         fatalError("Cannot schedule tasks on an EventLoop that has already shut down.")
                     }
-                    fputs(
+                    printError(
                         """
                         ERROR: Cannot schedule tasks on an EventLoop that has already shut down. \
                         This will be upgraded to a forced crash in future SwiftNIO versions.\n
-                        """,
-                        stderr
+                        """
                     )
                     return false
                 }