MEDIUM: cfgparse: acknowledge that proxy ID auto numbering starts at 2

MEDIUM: cfgparse: acknowledge that proxy ID auto numbering starts at 2

Each frontend/backend/listen proxies is assigned an unique ID. It can
either be set explicitely via 'id' keyword, or automatically assigned on
post parsing depending on the available values.

It was expected that the first automatically assigned value would start
at '1'. However, due to a legacy bug this is not the case as this value
is always skipped. Thus, automatically assigned proxies always start at
'2' or more.

To avoid breaking the current existing state, this situation is now
acknowledged with the current patch. The code is rewritten with an
explicit warning to ensure that this won't be fixed without knowing the
current status. A new regtest also ensures this.