BUG/MINOR: proxy/server: Skip per-proxy/server post-check for disabled proxies

per-proxy and per-server post-check callback functions must be skipped for
disabled proxies because most of the configuration validity check is skipped for
these proxies.

This patch must be backported as far as 2.1.
diff --git a/src/haproxy.c b/src/haproxy.c
index 3e6fd18..2b5aa70 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -1958,6 +1958,9 @@
 		struct post_proxy_check_fct *ppcf;
 		struct post_server_check_fct *pscf;
 
+		if (px->disabled)
+			continue;
+
 		list_for_each_entry(pscf, &post_server_check_list, list) {
 			for (srv = px->srv; srv; srv = srv->next)
 				err_code |= pscf->fct(srv);