Fix deserialization of config values broken by 69266cd6.

This should fix issue #1828 (closed).

This patch makes it explicit that we store arrays in the config as json encoded arrays, while we allow both json encoded and PHP serialized arrays to be deserialized correctly. Unless it's a brand new install, the existing data in the database will be PHP serialized.

I've also added a hardening measure in case we fall back to PHP unserialize, making sure we're not vulnerable to a PHP Object Injection attack. This means that deserializing arrays containing PHP objects will no longer work, but afaict we never do that anyways, so I don't think that should break anything.