vswitchd: Add unixctl command to dump all flows, including hidden ones

vswitchd: Add unixctl command to dump all flows, including hidden ones

Previously, the only way to query the flow table was to run "ovs-ofctl
dump-flows".  This returned most flows, but not those marked hidden by
secchan.  Hidden flows are setup by mechanisms such as in-band control,
since they must not be modified by users of the controller.  However,
when debugging problems on the switch, it is often useful to see what
the flow table is actually doing.  The new "bridge/dump-flows" command
added to ovs-appctl shows all flows being used by the OpenFlow stack.