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/github.com/oschwald/maxminddb-golang/mmap_unix.go

@@ -0,0 +1,16 @@
+//go:build !windows && !appengine && !plan9 && !js && !wasip1 && !wasi
+// +build !windows,!appengine,!plan9,!js,!wasip1,!wasi
+
+package maxminddb
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func mmap(fd, length int) (data []byte, err error) {
+	return unix.Mmap(fd, 0, length, unix.PROT_READ, unix.MAP_SHARED)
+}
+
+func munmap(b []byte) (err error) {
+	return unix.Munmap(b)
+}