MINOR: proxy: add a true list containing all proxies

MINOR: proxy: add a true list containing all proxies

We have global proxies_list pointer which is announced as the list of
"all existing proxies", but in fact it only represents regular proxies
declared on the config file through "listen, frontend or backend" keywords

It is ambiguous, and we currently don't have a straightforwrd method to
iterate over all proxies (either public or internal ones) within haproxy

Instead we still have to manually iterate over multiple lists (main
proxies, log-forward proxies, peer proxies..) which is error-prone.

In this patch we add a struct list member (8 bytes) inside struct proxy
in order to store every proxy (except default ones) within a global
"proxies" list which is actually representative for all proxies existing
under haproxy process, like we already have for servers.