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

[Drew Adams <drew.adams@oracle.com>]

Assigning the priority of the hl-line overlay as -50 breaks Emacs
convention and flies in the face of overlay behavior as documented.
(elisp) `Overlay Properties' says:

 If you want to specify a priority value, use either `nil' (or zero),
 or a positive integer.  Any other value has undefined behavior.
      ^^^^^^^^           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Maybe that policy and guideline needs to be modified in light of the
chosen fix for bug #16192?  Maybe it should now say what negative
values mean (what the behavior is).

Note that not only has Emacs Dev violated this past guideline, but
3rd-party code will now do the same.  To fix precisely this bug in
hl-line+.el, for example, I had to follow suit.

[Again, instead of reducing the priorities of multiple overlays case
by case as bugs are noticed, a better fix would be to raise the
region priority.  If the other, pre-existing overlays already have
priorities that make sense relative to each other, then it is only
the priority of the new, region overlay that needs to be established.]