Revamp client's Windows POSIX compatibility code

Revamp client's Windows POSIX compatibility code

Unlike POSIX sockets, Windows sockets are not file descriptors, but
"OS handles", with a completely separate set of functions.

However, Windows can create a file descriptor for a socket, and return
a file descriptor's underlying handle.  Use that instead of our gross
hacks to keep up the illusion that sockets are file descriptors.
Slightly dirty: we put file descriptors into fd_set.  Works because
both boil down to int.  Change w32_select(), w32_socket(),
w32_connect(), w32_recv(), w32_writev_socket(), w32_send() to take and
return only file descriptors, and map to sockets internally.  Replace
w32_close_socket() by w32_close(), and drop the close() macro hackery
that made tcp_connect(), host_connect() use w32_close_socket().  New
fd_is_socket().

Windows provides select()-like functions only for handles.  Because of
that, the client used a handle for reading script files, and stored it
in file descriptor input_fd.  Drop this dirty hack, use a file
descriptor instead.  Works because we can get its underlying handle.
Remove the dirty macro hackery that made play(), ring_from_file() and
doexecute() unwittingly work with a handle.  Remove w32_openhandle()
and w32_close_handle().  Replace w32_readv_handle() by w32_readv_fd().
Update w32_select().

Remove w32_openfd(), it's not really needed.

The old code stuffed WSA error codes into errno, which doesn't work.
Use new w32_set_winsock_errno() to convert & stuff.

Fix signed vs. unsigned warnings in Windows client.

Move the struct sigaction replacement next to the sigaction()
replacement.

Rename sysdep_init() to w32_sysdep_init() for consistency.