bug#16192: hl-line-mode: Current line highlighting takes precedence over region highlighting

[Stefan Monnier]

> It is the region overlay that is new.  It is its priority that
> needs fixing.

This narrow-minded "if a bug appear it must be in the new code" point of
view leads to unmaintainable messes of hacks upon hacks.

hl-line's overlay is a "background" highlight (not in the "background
color" sense, but in a "background noise" sense), so it makes a lot of
sense to give it a low priority.


        Stefan