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/modernc.org/libc/libc_unix2.go

@@ -0,0 +1,26 @@
+// Copyright 2024 The Libc 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 illumos
+
+package libc // import "modernc.org/libc"
+
+import (
+	"modernc.org/libc/sys/types"
+)
+
+// ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);
+func Xrecvmsg(t *TLS, sockfd int32, msg uintptr, flags int32) types.Ssize_t {
+	if __ccgo_strace {
+		trc("t=%v sockfd=%v msg=%v flags=%v, (%v:)", t, sockfd, msg, flags, origin(2))
+	}
+	panic(todo(""))
+	//	n, _, err := unix.Syscall(unix.SYS_RECVMSG, uintptr(sockfd), msg, uintptr(flags))
+	//	if err != 0 {
+	//		t.setErrno(err)
+	//		return -1
+	//	}
+	//
+	// return types.Ssize_t(n)
+}