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/dev_zos.go

@@ -0,0 +1,28 @@
+// Copyright 2020 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 zos && s390x
+
+// Functions to access/create device major and minor numbers matching the
+// encoding used by z/OS.
+//
+// The information below is extracted and adapted from <sys/stat.h> macros.
+
+package unix
+
+// Major returns the major component of a z/OS device number.
+func Major(dev uint64) uint32 {
+	return uint32((dev >> 16) & 0x0000FFFF)
+}
+
+// Minor returns the minor component of a z/OS device number.
+func Minor(dev uint64) uint32 {
+	return uint32(dev & 0x0000FFFF)
+}
+
+// Mkdev returns a z/OS device number generated from the given major and minor
+// components.
+func Mkdev(major, minor uint32) uint64 {
+	return (uint64(major) << 16) | uint64(minor)
+}