MAJOR: cfgparse: make sure server names are unique within a backend

MAJOR: cfgparse: make sure server names are unique within a backend

There was already a check for this but there used to be an exception
that allowed duplicate server names only in case where their IDs were
explicit and different. This has been emitting a warning since 3.1 and
planned for removal in 3.3, so let's do it now. The doc was updated,
though it never mentioned this unicity constraint, so that was added.

Only the check for the exception was removed, the rest of the code
that is currently made to deal with duplicate server names was not
cleaned yet (e.g. the tree doesn't need to support dups anymore, and
this could be done at insertion time). This may be a subject for future
cleanups.