* lisp/textmodes/fill.el (current-fill-column): Make nil value of
'fill-column' obsolete. (Bug#22847)
(current-fill-column--has-warned): New variable to track warning.
* lisp/simple.el (do-auto-fill): Remove handling of nil return value
from 'current-fill-column'.
* etc/NEWS: Announce obsoletion of this usage.