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/sysvshm_linux.go

@@ -0,0 +1,20 @@
+// Copyright 2021 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 linux
+
+package unix
+
+import "runtime"
+
+// SysvShmCtl performs control operations on the shared memory segment
+// specified by id.
+func SysvShmCtl(id, cmd int, desc *SysvShmDesc) (result int, err error) {
+	if runtime.GOARCH == "arm" ||
+		runtime.GOARCH == "mips64" || runtime.GOARCH == "mips64le" {
+		cmd |= ipc_64
+	}
+
+	return shmctl(id, cmd, desc)
+}