bgpd: Show why the prefix is inaccessible in show commands

bgpd: Show why the prefix is inaccessible in show commands

```
donatas-pc# show ip bgp 100.100.100.0/24 longer-prefixes
BGP table version is 13, local router ID is 10.10.10.10, vrf id 0
Default local pref 100, local AS 65000
Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes:  i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

   Network          Next Hop            Metric LocPrf Weight Path
   100.100.100.0/24 0.0.0.0                  0         32768 i

Displayed  1 routes and 15 total paths
donatas-pc# show ip bgp 100.100.100.0/24
BGP routing table entry for 100.100.100.0/24, version 0
Paths: (1 available, no best path)
  Not advertised to any peer
  Local
    0.0.0.0 (inaccessible, import-check enabled) from 0.0.0.0 (10.10.10.10)
      Origin IGP, metric 0, weight 32768, invalid, sourced, local
      Last update: Tue Oct  4 11:31:44 2022
donatas-pc# show ip bgp 100.100.100.0/24 json
{
  "prefix":"100.100.100.0\/24",
  "version":0,
  "paths":[
    {
      "aspath":{
        "string":"Local",
        "segments":[
        ],
        "length":0
      },
      "origin":"IGP",
      "metric":0,
      "weight":32768,
      "valid":false,
      "version":0,
      "sourced":true,
      "local":true,
      "lastUpdate":{
        "epoch":1664872304,
        "string":"Tue Oct  4 11:31:44 2022\n"
      },
      "nexthops":[
        {
          "ip":"0.0.0.0",
          "hostname":"donatas-pc",
          "afi":"ipv4",
          "accessible":false,
          "importCheckEnabled":true,
          "used":true
        }
      ],
      "peer":{
        "peerId":"0.0.0.0",
        "routerId":"10.10.10.10"
      }
    }
  ]
}
donatas-pc#
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>