Make the "All" setting for large-newsgroup-initial in Gnus work

* lisp/gnus/gnus-sum.el (gnus-articles-to-read): Use it.
(gnus-summary-insert-old-articles): Ditto.

* lisp/gnus/gnus.el (large-newsgroup-initial): Make the "All"
setting work by using a special symbol, instead of nil which is
indistinguishable from not being present (bug#38466).
1 job for master in 56 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #6804
test-all

00:56:17

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1