Rework the handling of storage/auth backends and config

The legacy way of transmitting backend settings to the client
for initial core configuration was pretty weird, especially regarding
the setupKeys/setupDefaults mess that required guesswork on the
client side as of the property types. With the introduction of
authenticators, an opportunity presented itself to improve this.

Upon fixing this, I realized that lots of pretty crazy legacy code
was involved, and partially even duplicated for authenticator
handling. I could not keep myself from cleaning that up.
In the process, the affected parts of the code were modernized too,
leveraging Modern C++ and the STL where possible.

The setupKeys/setupDefaults properties of the backend info structs
are now deprecated and replaced by a setupData list of field IDs,
translatable display names, and default values/types. To keep
legacy clients working, the old properties are still transmitted
until the next protocol break.