MEDIUM: cfgparse: warn about deprecated use of duplicate server names

MEDIUM: cfgparse: warn about deprecated use of duplicate server names

As discussed below, there are too many problems and limitations caused
by still supporting duplicate server names. That's already particularly
complicated and dissuasive to use since it requires these servers to
have explicit IDs to be accept. Let's now warn on any duplicate, even
with explicit IDs and remind that this will become forbidden in 3.3.

Link: https://www.mail-archive.com/haproxy@formilux.org/msg45185.html