Make confparse able to handle multiple config_format_t objects at once

Before we can move on to legacy/trac#30866 (moved) and have the responsibility for configuration formats spread across modules, we need to have the confparse code able to handle multiple configuration objects and formats as if they were one.

I'm going to do this by introducing a "configuration manager" type that knows about a bunch of config_format_t objects. The configuration objects will be collected under a single top-level configuration object. (This approach lets us keep backward compatibility with a lot of the existing options management code.)