Postpone junk processing (closes SF #2945712). Patch submitted by Ted

Postpone junk processing (closes SF #2945712). Patch submitted by Ted
Phelps and refined by Bill Wohler.

* mh-e.el (mh-blacklist, mh-whitelist): New variables.
(mh-whitelist-preserves-sequences-flag): New option.
(mh-before-commands-processed-hook): Update documentation.
(mh-blacklist-msg-hook, mh-whitelist-msg-hook): New hooks.
(mh-folder-blacklisted, mh-folder-whitelisted): New faces.
* mh-folder.el (mh-folder-message-menu): Add "Junk" to "Undo."
(mh-folder-font-lock-keywords): Add regexps for blacklisted and
whitelisted messages.
(mh-folder-mode): Add mh-blacklist and mh-whitelist variables.
(mh-execute-commands): Update documentation.
(mh-undo, mh-outstanding-commands-p, mh-process-commands)
(mh-delete-a-msg, mh-refile-a-msg, mh-undo-msg): Handle blacklisted
and whitelisted messages.
* mh-junk.el (mh-junk-blacklist, mh-junk-whitelist): Update to put
messages in blacklist and whitelist respectively for latter
processing.
(mh-blacklist-a-msg, mh-junk-whitelist-a-msg): New function to support
previous functions.
(mh-junk-blacklist-disposition): New function.
(mh-junk-process-blacklist, mh-junk-process-whitelist): New functions
that perform the blacklisting and whitelisting respectively that used
to be performed by mh-junk-blacklist and mh-junk-whitelist.
* mh-scan.el (mh-scan-blacklisted-msg-regexp)
(mh-scan-whitelisted-msg-regexp): New scan line regexps.
(mh-scan-good-msg-regexp): Add B and W characters to regexp.
(mh-scan-cmd-note-width): Update documentation.
(mh-note-blacklisted, mh-note-whitelisted): New scan line characters.
* mh-search.el (mh-index-execute-commands): Handle blacklisted and
whitelisted messages.