This change add supports for pipes on POSIX and Windows (when I/O
completion ports are available). For example, to create and use a
connected pair of pipe objects:
asio::readable_pipe read_end;
asio::writable_pipe write_end;
asio::connect_pipe(read_end, write_end);
write_end.async_write_some(my_write_buffer,
[](error_code e, size_t n)
{
// ...
});
read_end.async_read_some(my_read_buffer,
[](error_code e, size_t n)
{
// ...
});
