Simplified the code for the alert() function.

  Tested it with UTF-8 string too.
  comment-update
  User our mime_type primitive for attaching files.

  Rather than adding attachments as application/octet-stream.
  BUGFIX:  The mime_type parsing is improved.

  Previously we'd parse /etc/mime_types, but not tokenize in the
 case of multiple matches.  This would lead to image/jpg being
 stored in our table with the suffix:
        jpg jpeg jpe

  We now tokenize so that:

        mime[jpg]  -> image/jpg
        mime[jpeg] -> image/jpg
        mime[jpe]  -> image/jpg

  This is the expected result.  (Spotted after updating the
 attachment handling to use the correct type.)
  Removed trailing ";" - as per style-guidelines.

  Found by "make style".
  Removed misleading comments.
  Removed TODO - which I don't intend to change.

  Because I'm not seeing a real gain.
  Added roadmap.
  Added message_offset().

  Similar to index_offset() / maildir_offset().  Not sure it
 will be useful, but the absence was annoying me.