bgpd: Show PfxSnt for `show bgp <afi> <safi>` command

bgpd: Show PfxSnt for `show bgp <afi> <safi>` command

JSON output was already in, just printing in CLI as well.

```
root@exit1-debian-9:~# vtysh -c 'show bgp ipv4 summary'

IPv4 Unicast Summary:
BGP router identifier 10.10.10.1, local AS number 65001 vrf-id 0
BGP table version 100003
RIB entries 200005, using 37 MiB of memory
Peers 1, using 21 KiB of memory

... MsgRcvd   MsgSent   TblVer  InQ OutQ  Up/Down State/PfxRcd   PfxSnt
...      64       157        0    0    0 00:01:18            0   100003

Total number of neighbors 1
```

JSON:

```
root@exit1-debian-9:~# vtysh -c 'show bgp ipv4 summary json' \ |
> jq '.ipv4Unicast.peers."192.168.0.2".pfxSnt'
100003
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>