MEDIUM: log: Use linked lists for loggers

This patch settles the 2 loggers limitation.
Loggers are now stored in linked lists.

Using "global log", the global loggers list content is added at the end
of the current proxy list. Each "log" entries are added at the end of
the proxy list.

"no log" flush a logger list.
diff --git a/src/frontend.c b/src/frontend.c
index 19980c0..a32018c 100644
--- a/src/frontend.c
+++ b/src/frontend.c
@@ -144,7 +144,7 @@
 	}
 
 	if ((s->fe->mode == PR_MODE_TCP || s->fe->mode == PR_MODE_HTTP)
-	    && (s->fe->logfac1 >= 0 || s->fe->logfac2 >= 0)) {
+	    && (!LIST_ISEMPTY(&s->fe->logsrvs))) {
 		if (likely(s->fe->to_log)) {
 			/* we have the client ip */
 			if (s->logs.logwait & LW_CLIP)