bug#5026: 23.1; add-hook buffer local vs single func

[Kevin Ryde]

Chong Yidong <cyd@stupidchicken.com> writes:
>
> Still, as long as we're still supporting the obsolete format, it's easy
> enough to fix run-hooks to recognize it; I've checked in such a fix.

Good stuff.

As an idea, maybe the bit of add-hook that upgrades a single func to a
list could be split out and forcibly applied in places that care, like
customize or whatever.  I wouldn't mind if even run-hooks upgraded the
value.  This could preserve compatibility for now, and be a half step
towards breaking compatibility later.