MEDIUM: config: only warn if stats are attached to multi-process bind directives

Some users want to have a stats frontend with one line per process, but while
100% valid and safe, the config parser emits a warning. Relax this check to
ensure that the warning is only emitted if at least one of the listeners is
bound to multiple processes, or if the directive is placed in a backend called
from multiple processes (since in this case we don't know if it's safe).
(cherry picked from commit eb791e03b5c5abfddb24a439fa6434788db026b7)
diff --git a/src/cfgparse.c b/src/cfgparse.c
index f3907bf..5668393 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -7189,8 +7189,19 @@
 
 		if (nbproc > 1) {
 			if (curproxy->uri_auth) {
-				Warning("Proxy '%s': in multi-process mode, stats will be limited to process assigned to the current request.\n",
-				        curproxy->id);
+				int count, maxproc = 0;
+
+				list_for_each_entry(bind_conf, &curproxy->conf.bind, by_fe) {
+					count = popcount(bind_conf->bind_proc);
+					if (count > maxproc)
+						maxproc = count;
+				}
+				/* backends have 0, frontends have 1 or more */
+				if (maxproc != 1)
+					Warning("Proxy '%s': in multi-process mode, stats will be"
+					        " limited to process assigned to the current request.\n",
+					        curproxy->id);
+
 				if (!LIST_ISEMPTY(&curproxy->uri_auth->admin_rules)) {
 					Warning("Proxy '%s': stats admin will not work correctly in multi-process mode.\n",
 					        curproxy->id);