vsh: Rework logic for picking which argument is to be completed

vsh: Rework logic for picking which argument is to be completed

Currently the code decides which option to complete by looking into the
input string and trying to infer it based on whether we are at the
end position as we truncate the string to complete to the current cursor
position.

That basically means that only the last-parsed option will be up for
completion.

Replace the logic by remembering which is the last option rather than
using two different position checks and base the completion decision on
that and the actual value of the last argument (see comment).

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>