MINOR: cfgparse: move proxy post-init in a dedicated function

A lot of proxies initialization code is delayed on post-parsing stage,
as it depends on the configuration fully parsed. This is performed via a
loop on proxies_list.

Extract this code in a dedicated function proxy_finalize(). This patch
will be useful for dynamic backends creation.

Note that for the moment the code has been extracted as-is. With each
new features, some init code was added there. This has become a giant
loop with no real ordering. A future patch may provide some cleanup in
order to reorganize this.

include/haproxy/proxy.h

@@ -97,6 +97,7 @@ int resolve_stick_rule(struct proxy *curproxy, struct sticking_rule *mrule);
 void free_stick_rules(struct list *rules);
 void free_server_rules(struct list *srules);
 int proxy_init_per_thr(struct proxy *px);
+int proxy_finalize(struct proxy *px, int *err_code);
 
 /*
  * This function returns a string containing the type of the proxy in a format