bgpd: don't make any assumptions about the size of an enum

bgpd: don't make any assumptions about the size of an enum

The size of an enum is compiler dependent and thus we shouldn't use
enums inside structures that represent fields of a packet.

Problem detected by the 'test_capability' unit test.

The problem was not apparent before because the 'iana_safi_t' enum didn't
exist and 'safi_t' was a typedef to uint8_t. Now we have two different
enums, 'iana_afi_t' and 'iana_safi_t', and both need to be encoded in
different ways on the wire (2 bytes vs 1 byte).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>