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/AUTHORS

@@ -0,0 +1,23 @@
+# This file lists authors for copyright purposes.  This file is distinct from
+# the CONTRIBUTORS files.  See the latter for an explanation.
+#
+# Names should be added to this file as:
+#     Name or Organization <email address>
+#
+# The email address is not required for organizations.
+#
+# Please keep the list sorted.
+
+Dan Kortschak <dan@kortschak.io>
+Dan Peterson <danp@danp.net>
+David Leadbeater <dgl@dgl.cx>
+Fabrice Colliot <f.colliot@gmail.com>
+Jan Mercl <0xjnml@gmail.com>
+Jason DeBettencourt <jasond17@gmail.com>
+Jasper Siepkes <jasper@siepkes.nl>
+Koichi Shiraishi <zchee.io@gmail.com>
+Marius Orcsik <marius@federated.id>
+Olivier Mengué <dolmen@cpan.org>
+Patricio Whittingslow <graded.sp@gmail.com>
+Scot C Bontrager <scot@indievisible.org>
+Steffen Butzer <steffen(dot)butzer@outlook.com>