vsh: Rework how option to complete is found

vsh: Rework how option to complete is found

The way that auto completion works currently is that user's input
is parsed, and then we try to find the first --option (in the
parsed structure) that has the same value as user's input around
where <TAB> was pressed. For instance, for the following input:

  virsh # command --arg1 hello --arg2 world<TAB>

we will see "world" as text that user is trying to autocomplete
(this is affected by rl_basic_word_break_characters which
readline uses internally to break user's input into individual
words) and find that it is --arg2 that user is trying to
autocomplete. So far so good, for this naive approach. But
consider the following example:

  virsh # command --arg1 world --arg2 world<TAB>

Here, both arguments have the same value and because we see
"world" as text that user is trying to autocomplete we would
think that it is --arg1 that user wants to autocomplete. This is
obviously wrong.

Fortunately, readline stores the current position of cursor (into
rl_point) and we can use that when parsing user's input: whenever
we reach a position that matches the cursor then we know that
that is the place where <TAB> was pressed and hence that is the
--option that user wants to autocomplete. Readline stores the
cursor position as offset (numbered from 1) from the beginning of
user's input. We store this input into @parser->pos initially,
but then advance it as we tokenize it. Therefore, what we need is
to store the original position too.

Thanks to Martin who helped me with this.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>