rename {pi_config} fields to {pi_cfg}

{pi_config} may be confused with the documented `PI_CONFIG'
environment variable, and we'll favor vowel-removal to be
consistent with our usage of object references.

The `pi_' prefix may stay in some places, for now; since a
separate namespace may come into this codebase for local/private
client-tooling.

For InboxIdle, we'll also remove an invalid comment about
holding a reference to the PublicInbox::Config object, too.