gpgolconfig: Consistently respect immutability of options

The option "Also with untrusted keys" was sometimes enabled when another
option was checked even if the corresponding config value
"autoencryptUntrusted" was marked as immutable.

The same happened for "autosecure" and "searchSmimeServers".

Now options that are marked as immutable are never enabled.

• GnuPG-bug-id: T8254