ovs-ctl: Make "force-reload-kmod" warn when DHCP clients must be restarted.

This should make it more obvious when the admin needs to restart a DHCP
client (or other daemon).  Without this, unless the admin carefully reads
the documentation, the first notice he gets about a need to restart the
DHCP client can easily be when the lease expires and the machine drops off
the network.

Bug #5391.
Tested-by: Gurucharan Shetty <gshetty@nicira.com>
Suggested-by: Duffie Cooley <dcooley@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>