MINOR: counters: Review conditions to increment counters from analysers

Now, for these counters, the following rules are followed to know if it must be
incremented or not:

  * if it exists for a frontend, the counter is incremented
  * if stats must be collected for the session's listener, if the counter exists
    for this listener, it is incremented
  * if the backend is already assigned, if the counter exists for this backend,
    it is incremented
  * if a server is attached to the stream, if the counter exists for this
    server, it is incremented

It is not hardcoded rules. Some counters are still handled in a different
way. But many counters are incremented this way now.
diff --git a/src/fcgi-app.c b/src/fcgi-app.c
index 2e01cdb..f7108c3 100644
--- a/src/fcgi-app.c
+++ b/src/fcgi-app.c
@@ -474,10 +474,11 @@
 
   rewrite_err:
 	_HA_ATOMIC_ADD(&sess->fe->fe_counters.failed_rewrites, 1);
-	if (sess->fe != s->be)
-		_HA_ATOMIC_ADD(&s->be->be_counters.failed_rewrites, 1);
+	_HA_ATOMIC_ADD(&s->be->be_counters.failed_rewrites, 1);
 	if (sess->listener->counters)
 		_HA_ATOMIC_ADD(&sess->listener->counters->failed_rewrites, 1);
+	if (objt_server(s->target))
+		_HA_ATOMIC_ADD(&__objt_server(s->target)->counters.failed_rewrites, 1);
   hdr_rule_err:
 	node = ebpt_first(&hdr_rules);
 	while (node) {