mail/squirrelmail: update to 1.4.23pre14764

Fix CVE-2018-8741 and more.

  - Added ability (and user preference) to return to message list
    after moving a message
  - Search enhancement: Added ability to search in more than one
    header without having to search the body
  - Add ability for saved drafts to indicate if they are a reply and
    if so, to which message, and mark that message as replied when
    the draft is finally sent
  - Added option to allow returning to the message one had been
    replying to after sending
  - Sanitize user-supplied attachment filenames (thanks to Florian
    Grunow for reporting this issue) [CVE-2018-8741]
  - Allow users who cannot edit their email address but who have
    multiple identities to edit all their identities

(taca)