net: zperf: allow TCP receiver to handle multiple connections

Currently the zperf_tcp_receiver can handle only one TCP connection
each time, modify the code to poll and handle multiple connections.

Take the occasion to unify the bind and listen part of the code
between ipv4 and ipv6 part using a structure introduced to handle
the multiple connections.

Now in case the zsock_recv fails, we can't stop every connection
and fail through the error label, so just print the error message
and report the failure through the callback.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>