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/CONTRIBUTORS

@@ -0,0 +1,27 @@
+# This file lists people who contributed code to this repository.  The AUTHORS
+# file lists the copyright holders; this file lists people.
+#
+# Names should be added to this file like so:
+#     Name <email address>
+#
+# Please keep the list sorted.
+
+Bjørn Wiegell <bj.wiegell@gmail.com>
+Dan Kortschak <dan@kortschak.io>
+Dan Peterson <danp@danp.net>
+David Leadbeater <dgl@dgl.cx>
+Fabrice Colliot <f.colliot@gmail.com>
+Jaap Aarts <jaap.aarts1@gmail.com>
+Jan Mercl <0xjnml@gmail.com>
+Jason DeBettencourt <jasond17@gmail.com>
+Jasper Siepkes <jasper@siepkes.nl>
+Koichi Shiraishi <zchee.io@gmail.com>
+Leonardo Taccari <leot@NetBSD.org>
+Marius Orcsik <marius@federated.id>
+Olivier Mengué <dolmen@cpan.org>
+Patricio Whittingslow <graded.sp@gmail.com>
+Roman Khafizianov <roman@any.org>
+Scot C Bontrager <scot@indievisible.org>
+Steffen Butzer <steffen(dot)butzer@outlook.com>
+W. Michael Petullo <mike@flyn.org>
+ZHU Zijia <piggynl@outlook.com>