BUG/MINOR: syslog: Request for more data if message was not fully received

In the syslog applet, when a message was not fully received, we must request
for more data by calling appctx_need_more_data() and not by setting
CF_READ_DONTWAIT flag on the request channel. Indeed, this flag is only used
to only try a read at once.

This patch could be backported as far as 2.4. On 2.5 and 2.4,
applet_need_more_data() must be replaced by si_cant_get().
diff --git a/src/log.c b/src/log.c
index da9766d..cadab22 100644
--- a/src/log.c
+++ b/src/log.c
@@ -3663,8 +3663,7 @@
 
 missing_data:
 	/* we need more data to read */
-	sc_oc(sc)->flags |= CF_READ_DONTWAIT;
-
+	applet_need_more_data(appctx);
 	return;
 
 missing_budget: