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.

The ncurses header is typically found at
/usr/include/curses.h.
Non-standard locations are accommodated by
setting --extra-lib-dirs and --extra-include-dirs.
For example, Debian installs the ncurses headers
(libncursesw5-dev) under /usr/include/ncursesw.
To build ncursesw on Debian, use

cabal install --extra-include-dirs=/usr/include/ncursesw
ncursesw: add type alias for wint_t

On the C side, `wint_t` is a typedef used by
many of the wide char curses operations.
Previously, the binding assumed that `wint_t` is
an unsigned int.
Issue #53 indicates that the assumption about the
underlying type of `wint_t` may not hold
(the user experiences a build failure because
`CInt` is provided where `CUInt` is expected).
This patch introduces a corresponding type alias `Wint_t`
on the Haskell side which is set to the appropriate type
by c2hs at compile time.