BUG/MINOR: cfgparse: detect incorrect overlap of same backend names

BUG/MINOR: cfgparse: detect incorrect overlap of same backend names

As reported below, it's possible to declare a backend then a proxy with
the same name, because for the proxy we check a frontend capability (the
first one to be tested):

   backend b
   listen b
        bind :8888

Let's check the two capabilities in this case and not just the frontend.

Better not backport this, as there's a risk of breakage of existing
setups that work by accident. It might make sense to report them as
diag warnings though.

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