Reduce allout.el's pollution of the namespace.

* lisp/allout.el (allout-set-regexp): Rename from set-allout-regexp.
Keep old name as obsolete alias.
(allout-produce-mode-menubar-entries, allout-nullify-prefix-data)
(allout-solicit-char-in-string)
(allout-count-trailing-whitespace-region, allout-regexp-sans-escapes):
Rename to add an "allout-" prefix.