python: Break unixctl implementation into registry, client, and server.

python: Break unixctl implementation into registry, client, and server.

I wish to add some unixctl commands to the Python vlog module.  However,
importing ovs.unixctl in ovs.vlog creates a circular dependency, because
ovs.unixctl imports ovs.vlog already.  The solution, in this commit, is to
break the unixctl module into three parts: a register (ovs.unixctl) that
does not depend on ovs.vlog, and client (ovs.unixctl.client) and server
(ovs.unixctl.server) modules that do.  This breaks the circular dependency.

Signed-off-by: Ben Pfaff <blp@nicira.com>