bgpd: Add new configuration cli for graceful restart.

bgpd: Add new configuration cli for graceful restart.

There is support to configure graceful restart timer. This is the
time to wait to delete stale routes before a BGP open message is
received.

bgp graceful-restart restart-time <1-3600>
no bgp graceful-restart [<1-255>]

* bgpd/bgp_vty.c
  * Define command strings for above CLI
* bgpd/bgpd.c
  * bgp_config_write(): Output graceful restart-time configuration

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
Tested-by: NetDEF CI System <cisystem@netdef.org>