Move Windows socket stuff out off posixio.c into w32sockets.c

Move Windows socket stuff out off posixio.c into w32sockets.c

This is so we can avoid linking utilities with socket libraries (see
commit 8b778634).

When using sockets, we need to replace close(), because Windows'
close() can't cope with socket file descriptors.  But replacing it
always would pull in the socket stuff again.  Define close() to call
function pointer w32_close_function, which is initially _close.
Rename posix_close() to w32_close_maybe_socket().  Make new
w32_socket_init() put it in w32_close_function.

Same for read() and write(): define read(), write() to call function
pointers w32_read_function, w32_write_function, initially _read(),
_write(); rename posix_read(), posix_write() to
w32_read_maybe_socket(), w32_write_maybe_socket(), and put them into
w32_read_function, w32_write_function in w32_socket_init().

Also call WSAStartup() there, and use that from loc_NTInit().
WSACleanup() now belongs next to w32_socket_init().  Don't bother,
just drop it, along with loc_NTTerm().