(allout-v18/19-file-var-hack): Obsolete, remove.

(allout-v18/19-file-var-hack): Obsolete, remove.

(allout-mode): Argument "toggle" => "force".

Refine the docstring.

Remove special provisions for reactivation, besides the 'force' argument.
Consolidate layout provisions coce directly into the activation condition
branch, now that we've removed those provisions.

(allout-unload-function): Explicitly activate the mode before deactivating,
if it's initially deactivated.

(allout-set-buffer-multibyte): Properly prevent byte-compiler warnings for
version of function used only where set-buffer-multibyte is unavailable.