ovs-ctl.in: Ability to save flows and kernel datapath config.

ovs-ctl.in: Ability to save flows and kernel datapath config.

Add a new command - "restart" to ovs-ctl. Calling this command
will save and restore the Openflow flows on each bridge while
stopping and starting the userspace daemons respectively.

Also, during a force-reload-kmod, save the flows and kernel datapath
configuration. Use the saved datapath configuration while readding
the kernel module and the flows while starting the userspace daemons.

Feature #13555.
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>