ovn-nbctl: Initial support for daemon mode.

ovn-nbctl: Initial support for daemon mode.

Make ovn-nbctl act as a unixctl server if we were asked to detach. This
turns ovn-nbctl into a long-lived process that acts a proxy for
interacting with NB DB. The main difference to regular mode of ovn-nbctl
is that in the daemon mode, a local copy of database contents has to be
obtained only once.

Just two unixctl commands are supported 'run' and 'exit'. The former can
be used to run any ovn-nbctl command or a batch of them as so:

  ovs-appctl -t ovn-nbctl run [OPTIONS] COMMAND [-- [OPTIONS] COMMAND] ...

Running commands that have not yet been converted to not use ctl_fatal()
will result in death of the daemon process. However, --monitor option
can be used to keep the daemon running.

Signed-off-by: Jakub Sitnicki <jkbs@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>