Alternative method for equalising line number handling.

Created by: JonRowe

For @myronmarston the actual bug is that --line_number is supposed to be a standalone filter (like :location) which turns out to have had a bug in it's handling. Location filtering wasn't affected because it was manaully replacing the filter elsewhere.

I'd recommend this over #1193 as it solves the problem rather than working round it.

Fixes #1137 (closed), alternate to #1193