Workaround for stupid callback scheduler.

There is no reak callback scheduler and previous behavior causes
bad things during hard congestion (like BGP hold timeouts).

Smart callback scheduler is still missing, but main loop was
changed such that it first processes all tx callbacks (which
are fast enough) (but max 4* per socket) + rx callbacks for CLI,
and in the second phase it processes one rx callback per
socket up to four sockets (as rx callback can be slow when
there are too many protocols, because route redistribution
is done synchronously inside rx callback). If there is event
callback ready, second phase is skipped in 90% of iterations
(to speed up CLI during congestion).

lib/socket.h

@@ -51,6 +51,7 @@ void sk_reallocate(sock *);		/* Free and allocate tbuf & rbuf */
 void sk_dump_all(void);
 int sk_set_ttl(sock *s, int ttl);	/* Set TTL for given socket */
 int sk_set_md5_auth(sock *s, ip_addr a, char *passwd);	/* Add or remove security associations for given passive socket */
+int sk_rx_ready(sock *s);
 
 static inline int
 sk_send_buffer_empty(sock *sk)