Store plClient password using platform-specific credential storage.

Add an abstracted password-storage system, and utilize it in plClient to replace the drive-based obfuscation.
Even if we fix things so that the client spoofs the MOULa uuid somehow and get Gehn (and TOC) to have true UUIDs again, we can't promise that each server's UUID is unique. Lots of folks have set up shards with the default UUID, which would cause MOULa credential mayhem. That's not mentioning any sort of UU style shard gorup popping up that uses the same client/data base (and therefore the same UUID) but separate auth.