bug#5042: 23.1; linum-mode gives incorrect line numbers with narrowed buffers

[Drew Adams]

> > there should be an explicit option to switch between the modes.
> I agree, and don't really  care which one is default

Yes, an option makes sense.

And it can happen that the same person wants both behaviors at different times.
For that, why not let `C-u linum-mode' use, in effect, the opposite of the
option value?

IOW, if you set the option to use absolute numbering (per the file lines), and
you use `C-u linum-mode', then you get relative numbering instead (per the
visible lines). If you then exit the mode and reenter it (without `C-u'), you
get the default numbering (i.e., per the option value - absolute numbering in
this case).