CLEANUP: cfgparse-listen: move "stats" to its own parser

CLEANUP: cfgparse-listen: move "stats" to its own parser

The keyword is now registered in cfg_kws and parsed by
proxy_parse_stats(), making it discoverable via "-dKcfg". It needs the
default proxy and takes it via the <defpx> argument.

Like for previous http-request rules, the "http-request stats" action
will now return only the first failure and not all of them at once.

Note that <acl_cond> was removed from cfg_parse_listen() to avoid a
build warning since it was the last part using it, we're getting
closer to the end!