Warn when the info API's windows pipe is outside the protected namespace

A program reading the info API opens the pipe by name and has no owner
check of its own, so for a name any local user could create first,
whoever did would be answering its queries. nebula refuses to serve a
held name and logs it, but the program cannot tell. The daemon now warns
at bind when the configured name is not under
\\.\pipe\ProtectedPrefix\Administrators\, and the example config and
CHANGELOG say to put it there.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

localapi/platform_windows.go

@@ -251,6 +251,19 @@ const DefaultSocketPath = protectedPrefix + "nebula-ctl"
 // pipePrefix is used as given. An override outside protectedPrefix is a name any
 // account could have created first; the owner check in dial is what protects a client
 // of one.
+// unprotectedNameWarning says what a pipe name outside protectedPrefix costs, or "" for
+// one under it. A program reading the info API opens the pipe by name and has no owner
+// check of its own, so for a name any local user could create first, whoever did
+// would be answering its queries; the daemon refuses to serve a held name and logs it,
+// but the program cannot tell. The default control pipe is under the prefix; an info
+// pipe is wherever the operator put it.
+func unprotectedNameWarning(path string) string {
+	if len(path) >= len(protectedPrefix) && strings.EqualFold(path[:len(protectedPrefix)], protectedPrefix) {
+		return ""
+	}
+	return "any local user can create a pipe by this name ahead of the daemon and answer in its place; name it under " + protectedPrefix
+}
+
 func checkSocketPath(key, value string) error {
 	if len(value) <= len(pipePrefix) || !strings.EqualFold(value[:len(pipePrefix)], pipePrefix) {
 		return fmt.Errorf(`%s must be a full pipe name under %s, got %q`, key, pipePrefix, value)