Implement POSIX_MADV_WILLNEED/POSIX_MADV_DONTNEED for newer OSes

Implement POSIX_MADV_WILLNEED/POSIX_MADV_DONTNEED for newer OSes

        * autoload.cc (DiscardVirtualMemory): Import.
        (PrefetchVirtualMemory): Import.
        * mmap.cc (posix_madvise): Actually implement POSIX_MADV_WILLNEED
        utilizing PrefetchVirtualMemory and POSIX_MADV_DONTNEED utilizing
        DiscardVirtualMemory on systems supporting them.
        * wincap.h (wincaps::has_broken_prefetchvm): New element.
        * wincap.cc: Implement above element throughout.
        (wincapc::init): Make sure has_broken_prefetchvm is only true on
        W10 under WOW64.
        * include/cygwin/version.h (CYGWIN_VERSION_DLL_MAJOR): Bump to 2003.
        (CYGWIN_VERSION_API_MINOR): Reset to 0.

        * new-features.xml (ov-new2.3): New section, document posix_madvise
        POSIX_MADV_WILLNEED/POSIX_MADV_DONTNEED change.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>