'--skeleton --posix-captures': use correct capture index when checking tags.

'--skeleton --posix-captures': use correct capture index when checking tags.

Instead of skipping fixed tags, skip orbit tags. For each orbit tag there is
a tag fixed on it, so if we check all fixed tags we'll surely check all orbit
tags. Anyway, with '--skeleton' all fixed tags correspond to orbit tags: other
fixed tags are forbidden with generic API (even zero-offset ones for now).