Mitigate formspec exploits by verifying that the formspec was shown to the user by...

Mitigate formspec exploits by verifying that the formspec was shown to the user by the server. (#6878)

This doesn't check the fields in anyway whatsoever so it should only be seen as a way to mitigate exploits, a last line of defense to make it harder to exploit bugs in mods, not as a reason to not do all the usually checks.