Nest: Add command to request graceful restart

When 'graceful down' command is entered, protocols are shut down
with regard to graceful restart. Namely Kernel protocol does
not remove routes and BGP protocol does not send notification,
just closes the connection.

sysdep/unix/unix.h

@@ -28,6 +28,7 @@ void cmd_reconfig_confirm(void);
 void cmd_reconfig_undo(void);
 void cmd_reconfig_status(void);
 void cmd_shutdown(void);
+void cmd_graceful_restart(void);
 
 #define UNIX_DEFAULT_CONFIGURE_TIMEOUT	300