Windows: Use overlapped I/O, CreateFile

Windows: Use overlapped I/O, CreateFile

Overlapped I/O is win32's asynchronous I/O mechanism, which allows
us to start an I/O operation and explicitly wait for it to finish at
a time of our choosing, such that we can simultaneously await other
events instead of blocking unconditionally.

  - Support for overlapped I/O is a per-HANDLE flag specified at
    file opening time, necessitating a switch to win32's CreateFile
    and away from the CRT's _open.

  - Wrap win32 file operations in POSIX-compatible functions, so
    that UNIX-OPEN, UNIX-READ, UNIX-WRITE, UNIX-CLOSE continue to
    work as before.  Under the hood, these now call our Lisp or C
    functions instead of versions from CRT.

  - For now, these functions still return and expect what passes as
    file descriptors in CRT.

  - INTERRUPT-THREAD is now capable of performing the interruption
    in a target thread blocked in socket I/O, indicated using an
    errno of "EINTR".  Minor changes in FD streams to retry the I/O
    operation explicitly in that case.

Does not yet include changes for console I/O, and instead still
falls back to _read and _write in that case.  Also not yet included
is interruptible non-overlapped I/O, e.g. for unnamed pipes.

Thanks to Anton Kovalenko.