[PDCurses] Add _ISPAD define to curses.h

[techtonik]

Hello,

Ncurses exposes _ISPAD flag in public API. PDCurses has _PAD flag that is
hidden inside curspriv.h, but it is often useful to abstract API and know
when to use wechochar() and when pechochar(), for example. It seems that
there is nothing bad in adding _ISPAD along with other window defines into
curses.h

-- 
--anatoly t.