cmdline: big rework and clean of cmdline library

cmdline: big rework and clean of cmdline library

The main change is that we use the same structure to define a token.
The old method (parse, complete_get_nb, complete_get_elt, get_help) is
deprecated. We now have (parse, complete_start, complete_iterate,
complete_end, help) that will allow to do more complex tokens, like
a file token with completion.

Signed-off-by: Olivier Matz <zer0@droids-corp.org>