zebra: Ignore most netlink notifications from ourselves

zebra: Ignore most netlink notifications from ourselves

The BPF filter was an exclusion list of netlink messages
we did not want to receive from our self.  The problem
with this is that the exclusion list was and will be
ever growing.  So switch the test around to an inclusion
list since it is shorter and not growing.  Right
now this is RTM_NEWADDR and RTM_DELADDR.

Change some of the debug messages to error messages
so that when something slips through and it is unexpected
during development we will see the problem.

Also try to improve the documentation about what
the filter is doing and leave some breadcrumbs for
future developers to know where to change code
when new functionality is added.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>