(mh-sub-folders-actual): Mention that folder must have been processed

(mh-sub-folders-actual): Mention that folder must have been processed
by mh-normalize-folder-name.
(mh-folder-completion-function): Handle completion of folders with
absolute names. Also, when flag is t, display complete folder name to
provide proper highlighting in Emacs 22 now that
minibuffer-completing-file-name is nil (closes SF #1470518).
(mh-folder-completing-read): No longer set
minibuffer-completing-file-name to t. This was causing "Can't set
current directory errors" when browsing absolute file names. Another
benefit of this change is that SPC can be used for completion
again (closes SF #1470518).