Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/golang.org/x/sys/unix/syscall_hurd.go

@@ -0,0 +1,30 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build hurd
+
+package unix
+
+/*
+#include <stdint.h>
+int ioctl(int, unsigned long int, uintptr_t);
+*/
+import "C"
+import "unsafe"
+
+func ioctl(fd int, req uint, arg uintptr) (err error) {
+	r0, er := C.ioctl(C.int(fd), C.ulong(req), C.uintptr_t(arg))
+	if r0 == -1 && er != nil {
+		err = er
+	}
+	return
+}
+
+func ioctlPtr(fd int, req uint, arg unsafe.Pointer) (err error) {
+	r0, er := C.ioctl(C.int(fd), C.ulong(req), C.uintptr_t(uintptr(arg)))
+	if r0 == -1 && er != nil {
+		err = er
+	}
+	return
+}