Fix message of multi-line occur regexps and multi-buffer header lines.

Fix message of multi-line occur regexps and multi-buffer header lines.
http://lists.gnu.org/archive/html/emacs-devel/2010-03/msg00457.html

* replace.el (occur-1): Don't display regexp if it is longer
than window-width.  Use `query-replace-descr' to display regexp.
(occur-engine): Don't display regexp in the buffer header for
multi-buffer occur.  Display a separate header line with total
match count and regexp for multi-buffer occur.
Use `query-replace-descr' to display regexp.