add unixctl to ovn-controller

[SamYaple]

This just add --unixctl option to the ovn-controller daemon to bring it inline with available options in other daemons.

[dceara]

Thanks for the contribution @SamYaple! It looks OK but we're missing an update to the documentation part. For example for ovn-northd we have this:

   Other Options
       --unixctl=socket
              Sets the name of the control socket on which program listens for
              runtime  management  commands  (see RUNTIME MANAGEMENT COMMANDS,
              below). If socket does not begin with /, it  is  interpreted  as
              relative  to  .  If  --unixctl  is  not used at all, the default
              socket is /program.pid.ctl, where pid is program’s process ID.

              On Windows a local named pipe is used to listen for runtime man‐
              agement  commands.  A  file  is  created in the absolute path as
              pointed by socket or if --unixctl is not used at all, a file  is
              created  as  program in the configured OVS_RUNDIR directory. The
              file exists just to mimic the behavior of a Unix domain socket.

              Specifying none for socket disables the control socket feature.

That's generated here:

ovn/northd/ovn-northd.8.xml

Lines 84 to 87 in 3d0edc7

	<h2>Other Options</h2>
	<xi:include href="lib/unixctl.xml"
	xmlns:xi="http://www.w3.org/2003/XInclude"/>
	<h3></h3>

Also, it's not mandatory, but to get the most attention from OVN maintainers it's probably faster if you post your change as a patch to the mailing list. The contribution guidelines are available here:
https://docs.ovn.org/en/latest/internals/contributing/submitting-patches.html

Thanks,
Dumitru