Make low-level message formatting functions take a struct buffered.

interface.c

@@ -130,9 +130,9 @@ flush_interface(char *ifname)
 /* This should be no more than half the hello interval, so that hellos
    aren't sent late.  The result is in milliseconds. */
 unsigned
-jitter(struct interface *ifp, int urgent)
+jitter(struct buffered *buf, int urgent)
 {
-    unsigned interval = ifp->buf.flush_interval;
+    unsigned interval = buf->flush_interval;
     if(urgent)
         interval = MIN(interval, 20);
     else

interface.h

@@ -143,7 +143,7 @@ if_up(struct interface *ifp)
 
 struct interface *add_interface(char *ifname, struct interface_conf *if_conf);
 int flush_interface(char *ifname);
-unsigned jitter(struct interface *ifp, int urgent);
+unsigned jitter(struct buffered *buf, int urgent);
 unsigned update_jitter(struct interface *ifp, int urgent);
 void set_timeout(struct timeval *timeout, int msecs);
 int interface_up(struct interface *ifp, int up);