Occur-mode multi-line match property gap filling

Occur-mode multi-line match property gap filling

When an occur-mode regexp matches across multiple lines, the spacing
prefixes inserted between each did not have the `occur-target`
property which is essential for jumping to the corresponding place in
the target buffer.  This prevented next-error and previous-error
from working.

* lisp/replace.el (occur-engine): Put the `occur-target` property on
the continuation prefix to avoid the gap.