Changed syntax of attaching filters to protocols to hopefully the final

Changed syntax of attaching filters to protocols to hopefully the final
version:

	EXPORT <filter-spec>	for outbound routes (i.e., those announced
				by BIRD to the rest of the world).
	IMPORT <filter-spec>	for inbound routes (i.e., those imported
				by BIRD from the rest of the world).

where <filter-spec> is one of:

	ALL			pass all routes
	NONE			drop all routes
	FILTER <name>		use named filter
	FILTER { <filter> }	use explicitly defined filter

For all protocols, the default is IMPORT ALL, EXPORT NONE. This includes
the kernel protocol, so that you need to add EXPORT ALL to get the previous
configuration of kernel syncer (as usually, see doc/bird.conf.example for
a bird.conf example :)).