(occur-accumulate-lines): Avoid incf and decf.

(occur-accumulate-lines): Avoid incf and decf.
(occur-engine-add-prefix): New function.
(occur-engine): Avoid using macrolet, incf and decf.
Use occur-engine-add-prefix instead.
Rename `l' to `lines' and `c' to `matches'.

(occur-engine, occur-mode-mouse-goto)
(occur-mode-find-occurrence, occur-mode-goto-occurrence)
(occur-mode-goto-occurrence-other-window)
(occur-mode-display-occurrence): A position is just a marker,
not a list.

(occur-revert-arguments):
Renamed from occur-revert-properties.  All uses changed.