bgpd: Fix and enhance 'show neighbors graceful-restart'

Ensure that the 'show bgp neighbors graceful-restart' command works for
peers in a VRF (BGP instance) as well as doesn't require the incorrect
AFI parameter.

Also, refine the code and the command output such that this is a subset
of the 'show bgp neighbors' output for the neighbor, both in the CLI
output and in JSON.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>

Testing Done:
1. Manual test with global and VRF peerings
2. precommit - job FRRouting#82 -- 2 failures are existing failures
3. bgp-min - job FRRouting#83 -- 2 failures are existing failures