ncursesw: include curses.h on all distributions

curses.h exposes wide character variants and types
when NCURSES_WIDECHAR=1
(which in turn is implied by _XOPEN_SOURCE_EXTENDED).
Assuming the above holds on all recent ncurses installs,
we can simplify mycurses.h by unconditionally including
only curses.h.
ncursesw: no probing for ncurses install in Setup

Figuring out where ncurses is actually installed
is a task best left to package vendors.
This patch removes all probe logic from Setup.hs.

We assume that ncurses is installed at
/usr/include/curses.h.
Non-standard installs are easily accommodated by
setting --extra-lib-dirs and --extra-include-dirs.
For example, Debian installs the ncurses headers
(libncursesw5-dev) at /usr/include/ncursesw.
To build ncursesw on Debian, use

    cabal install --extra-include-dirs=/usr/include/ncursesw
README: document prereqs for building ncursesw