MEDIUM: log: use a separate buffer for the header and for the message

Make sendmsg() use two vectors, one for the message header that is updated
by update_log_hdr() and one for the message buffer.
diff --git a/include/proto/log.h b/include/proto/log.h
index 0967ae8..14dfc5b 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -39,6 +39,7 @@
 extern char default_tcp_log_format[];
 extern char default_http_log_format[];
 extern char clf_http_log_format[];
+extern char *logheader;
 extern char *logline;