MAJOR: cfgparse: turn the same proxy name warning to an error

MAJOR: cfgparse: turn the same proxy name warning to an error

As warned since 3.1, it's no longer permitted to have a frontend and
a backend under the same name. This causes too many designation issues,
and causes trouble with stick-tables as well. Now each proxy name is
unique.

This commit only changes the check to return an error. Some code parts
currently exist to find the best candidates, these will be able to be
simplified as future cleanup patches. The doc was updated.