io.files.windows: new generic words drain and refill, like in io.backend.unix

The purpose of these words is to abstract out the pushing of the bytes
to the ports so that you can insert the ssl layer in between. Exactly
like how drain and refill are specialized on ssl-handle in
io.sockets.secure.unix.
io.sockets.secure.windows: reading and writing from ssl sockets

Unified the drain and refill generics and put their definition in
io.files. They are now used by both the windows and unix ssl backend
for io. Windows ssl kind of works now, but the error cases are not
implemented correctly.