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/README.md

@@ -0,0 +1,9 @@
+# libc
+
+[![LiberaPay](https://liberapay.com/assets/widgets/donate.svg)](https://liberapay.com/jnml/donate)
+[![receives](https://img.shields.io/liberapay/receives/jnml.svg?logo=liberapay)](https://liberapay.com/jnml/donate)
+[![patrons](https://img.shields.io/liberapay/patrons/jnml.svg?logo=liberapay)](https://liberapay.com/jnml/donate)
+
+[![Go Reference](https://pkg.go.dev/badge/modernc.org/libc.svg)](https://pkg.go.dev/modernc.org/libc)
+
+Package libc is a partial reimplementation of C libc in pure Go.