bgpd: Add to neighbor prefix-counts the count of best path selected

bgpd: Add to neighbor prefix-counts the count of best path selected

When we have a prefix that has been selected, note that that
particular flag has been set and give that information to the
end user.

eva# show bgp ipv4 uni neighbors 192.168.161.131 prefix-counts
Prefix counts for 192.168.161.131, IPv4 Unicast
PfxCt: 814246

Counts from RIB table walk:

              Adj-in: 0
              Damped: 0
             Removed: 0
             History: 0
               Stale: 0
               Valid: 814246
             All RIB: 814246
       PfxCt counted: 814246
 PfxCt Best Selected: 0
             Useable: 814246
eva# show bgp ipv4 uni neighbors 192.168.161.2 prefix-counts
Prefix counts for 192.168.161.2, IPv4 Unicast
PfxCt: 814070

Counts from RIB table walk:

              Adj-in: 0
              Damped: 0
             Removed: 0
             History: 0
               Stale: 0
               Valid: 814070
             All RIB: 814070
       PfxCt counted: 814070
 PfxCt Best Selected: 814070
             Useable: 814070

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