aio: change to use unix io, not stdio

This changes especially makes buffered I/O work
with non-blocking channels.

- separate read and write buffering
- support for timeout on blocking read
- read/write on same channel in event loop with buffering
- read buffer is the same across read, gets, copyto
- autoflush non-blocking writes via event loop
- copyto can now copy to any filehandle-like command
- add some copyto tests

Signed-off-by: Steve Bennett <steveb@workware.net.au>