lib, bgpd: Create iana_afi.h for storing iana_afi/safi enums

lib, bgpd: Create iana_afi.h for storing iana_afi/safi enums

The iana_afi_t and iana_safi_t were being created in zebra.h
and zebra.h is a bit of a dumping ground.  When the iana_afi2str and
iana_safi2str functions were created, it was correctly pointed out
that we should just use the internal afi_t and safi_t 2str functions
but to do that we would need to include prefix.h in zebra.h.  Which
really is not the right thing to do.  This tells us that we need
to break out this code into it's own header.

Move to iana_afi.h the enums and specific functions and remove
from zebra.  Convert to using the afi2str and safi2str functions.

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