(hide-ifdef-shadow): New option.

(hide-ifdef-shadow): New option.
(hide-ifdef-shadow): New face.
(hide-ifdef-toggle-shadowing): New function to toggle between
shadowing and making code invisible.
(hide-ifdef-mode-submap): Add binding for
hide-ifdef-toggle-shadowing.
(hide-ifdef-mode-menu): Add entry for
hide-ifdef-toggle-shadowing.
(hide-ifdef-region-internal): Give new overlay hide-ifdef
property.  Shadow text when hide-ifdef-shadow is non-nil.
(hif-show-ifdef-region): Remove overlays with hide-ifdef
property set.
(hif-hide-line): Use when instead of if.
(hide-ifdef-initially, hide-ifdef-read-only, hide-ifdef-lines):
Remove unneeded * from doc-strings.