bug#56682: Fix the long lines font locking related slowdowns

[Gregory Heytings]

> > > > Adding such a variable only two weeks after locked narrowing has been 
> > > > introduced means that modes will have little incentive to adapt to 
> > > > that stronger constraint, if they can "fix" whatever problems that 
> > > > constraint might cause by setting that variable to nil in their 
> > > > initialization hooks.
> > >
> > > Modes can do that already by changing long-line-threshold to nil, 
> > > don't they?
> >
> > They can indeed, but that variable is documented as "for debugging 
> > purposes" only.
>
> That never stopped anyone, IME.

At least they cannot tell the haven't been warned...

Can this change at least wait a few days?  I'm working on the next 
iteration of the code, I'll see if I can include such a feature.