samples: net: echo_async_select: Use read()/write() if possible

If build with full POSIX API, use read()/write() instead of
recv()/send() calls for sockets.

We have read()/write() support for a while, but no samples/tests
actually performed at least a build test for it (so it will be
done now).

Fixes: #25407
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>