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>
diff --git a/bgpd/bgp_open.h b/bgpd/bgp_open.h
index 2b1382d..6233375 100644
--- a/bgpd/bgp_open.h
+++ b/bgpd/bgp_open.h
@@ -82,6 +82,7 @@
 #define CAPABILITY_CODE_DYNAMIC_LEN     0
 #define CAPABILITY_CODE_RESTART_LEN     2 /* Receiving only case */
 #define CAPABILITY_CODE_AS4_LEN         4
+#define CAPABILITY_CODE_ORF_LEN         5
 
 /* Cooperative Route Filtering Capability.  */