(c-scan-conditionals): A new function like c-forward-conditionals, but it

(c-scan-conditionals): A new function like c-forward-conditionals, but it
doesn't move point and doesn't set the mark.
(c-up-conditional, c-up-conditional-with-else, c-down-conditional)
(c-down-conditional-with-else, c-backward-conditional)
(c-forward-conditional): Refactor to use c-scan-conditionals.