bgpd: Allow specification of AS 0 for rpki commands

bgpd: Allow specification of AS 0 for rpki commands

RFC-7607 specifically calls out the allowed usage
of AS 0 to signal that the a particular address is
not in use and should be guarded against.  Add
the ability to specify this special AS in the rpki
commands.

eva# show rpki  as-number 0
RPKI/RTR prefix table
Prefix                                   Prefix Length  Origin-AS
2.57.180.0                                  22 -  24   0
2.58.144.0                                  22 -  22   0
2.59.116.0                                  24 -  24   0
4.42.228.0                                  22 -  22   0
5.57.80.0                                   22 -  22   0
<snip>
2a13:df87:b400::                            38 -  38   0
2a13:df84::                                 32 -  32   0
2630::                                      16 -  16   0
Number of IPv4 Prefixes: 1166
Number of IPv6 Prefixes: 617

eva# show rpki prefix 2630::/16 0
Prefix                                   Prefix Length  Origin-AS
2630::                                      16 -  16   0
eva#

Fixes: #15778
Signed-off-by: Donald Sharp <sharpd@nvidia.com>