new pattern modifiers ~a and %a to search all address fields of the envelope

[vinc17]

The attached patch adds new pattern modifiers ~a and %a to match messages which contain EXPR (or a member of GROUP) in some envelope address field, i.e. one of:

  ADDRESS *return_path;
  ADDRESS *from;
  ADDRESS *to;
  ADDRESS *cc;
  ADDRESS *bcc;
  ADDRESS *sender;
  ADDRESS *reply_to;
  ADDRESS *mail_followup_to;