FindCurses: Overhaul Curses and NCurses search logic (#15011)

Produce a more consistent result by finding only a single include
directory and reporting which headers may be included from it.  The
previous search for each header separately might find pieces from
separate and incompatible packages.

While at it, provide the CURSES_INCLUDE_DIRS result variable to be
consistent with other modules.
Merge topic 'FindCurses-overhaul' into next

6c8364e6 FindCurses: Overhaul Curses and NCurses search logic (#15011)