lib, bgpd: Fixup afi_t to be an enum and cleanup zebra.h

lib, bgpd: Fixup afi_t to be an enum and cleanup zebra.h

This code change does two things:

1) Removes ZEBRA_AFI_XXX #defines since they were redundant information
2) Switches afi_t to an enumerated type so that the compiler
can do a bit more compile time checking.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
(cherry picked from commit f3cfc46450cccc5ac035a5a97c5a1a5484205705)

Conflicts:
	bgpd/bgp_open.c
	bgpd/bgp_open.h
	bgpd/bgp_routemap.c