Changed packet drivers from services to plain processes.

Now tcpip_output() is a function pointer that is supposed to be set via the macro tcpip_set_outputfunc(). Packet drivers do so on process startup.

Thus if there are several packet drivers in a Contiki system the one started last is the one actually used. This behaviour is especially useful for the 'IP forwarding' "meta" packet driver.

doc/examples.txt

@@ -1,10 +1,7 @@
 /** \example example-program.c */
-/** \example example-service.c */
-/** \example example-service.h */
-/** \example example-use-service.c */
 /** \example example-pollhandler.c */
 /** \example example-list.c */
-/** \example example-packet-service.c */
+/** \example example-packet-drv.c */
 /** \example example-psock-server.c */
 
 /** \example test-abc.c */