CLEANUP: config: detect double registration of a config section

CLEANUP: config: detect double registration of a config section

In an effort to make the config parser more robust, we should validate
that everything we register is not already registered. Most cfg_register_*
functions unfortunately return void and just perform a LIST_ADDQ(), so they
will have to change for this. At least cfg_register_section() does perform
a bit of checks and is easy to check for such errors, so let's start with
this one. Future patches will definitely have to focus on the remaining
functions and ensure unicity of all config parsers.