ovs-appctl-comp: Refine the master for ovs-appctl completion.

After the hackathon, the best design for the ovs-appctl completion
seems to be as follow:

1. An extraction script extracts the sub-commands and options from
   the manpage.
2. A helper script which takes the command line string so far as
   input and returns the all available completions.
3. A generic compgen function that invokes the helper script/

Signed-off-by: Yeqiao Wang <ee07b291@gmail.com>