bgpd: BGP tcp session failed to apply GR configuration on the transferred

bgpd: BGP tcp session failed to apply GR configuration on the transferred
bgp tcp connection.

When the BGP peer is configured between two bgp routes  both routers would create
peer structure , when they receive each other’s open message. In this event both
speakers, open duplicate TCP sessions and send OPEN messages on each socket
simultaneously, the BGP Identifier is used to resolve which socket should be closed.
If BGP GR is enabled the old tcp session is dumped and the new session is retained.
So while this transfer of connection is happening, if all the bgp gr config
is not migrated to the new connection, the new bgp gr mode will never get applied.
Fix Summary:
1.  Replicate GR configuration from the old session to the new session in bgp_accept().
2.  Replicate GR configuration from stub to full-fledged peer in bgp_establish().
3.  Disable all NSF flags, clear stale routes (if present), stop  restart & stale timers
    (if they are running) when the bgp GR mode is changed to “Disabled”.
4.  Disable R-bit in cap, if it is not set the received open message.

Signed-off-by: Biswajit Sadhu <sadhub@vmware.com>