Alternative method for equalising line number handling.

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, alternate to #1193