FindCurses: Overhaul Curses and NCurses search logic

Simplify the logic by never finding "ncurses/ncurses.h" or
"ncurses/curses.h".  Instead always find either "ncurses.h" or
"curses.h" and report the include directory as ".../ncurses" if
necessary.  This also avoids asking applications to provide conditional
inclusion of both "ncurses/ncurses.h" and "ncurses.h".  Furthermore, it
produces a more consistent result by finding only a single location for
headers and never claiming that both "ncurses.h" and "ncurses/ncurses.h"
exist.  They might be 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

fcca1eb9 FindCurses: Overhaul Curses and NCurses search logic