ftruncate: Un-deprecate, concentrate on Win32 support.

ftruncate: Un-deprecate, concentrate on Win32 support.

* modules/ftruncate (Status, Notice): Remove sections.
(Depends-on): Add largefile.
* m4/ftruncate.m4 (gl_FUNC_FTRUNCATE): Drop failure message on
non-mingw platforms.
* lib/ftruncate.c: Remove code for the older platforms. For Win32,
include <io.h>.
* modules/perror-tests (Depends-on): Add ftruncate.
* doc/posix-functions/ftruncate.texi: Mention the MSVC problem and the
'ftruncate' module.