samples: sockets: echo_client/server: Build for minimal libc

With recent changes, sockets no longer depend on Newlib libc, so
let's have actual samples which demonstrate this. echo_client/
echo_server, which were ported from net_app samples, are converted.
The rest of socket samples are intended to be buildable on a
POSIX system (e.g. Linux), so they rather stay with more
full-fledged libc.

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