net: lib: zperf: fix internal UDP header definition

Fix the zperf UDP datagram header definition, as iperf introduced a
backwards incompatible format change in version 2.0.10 (August 2017).
```
struct UDP_datagram {
// used to reference the 4 byte ID number we place in UDP datagrams
// Support 64 bit seqno on machines that support them
    uint32_t id;
    uint32_t tv_sec;
    uint32_t tv_usec;
    uint32_t id2;
};
```

Update the header to the new format, with a Kconfig option to fall back
to the previous header definition.

The response decoding was testd with a nRF7002 client and
`iperf-2.2.1-win64.exe` server, with the output statistics struct now
containing the same information as reported on the PC server.

Signed-off-by: Jordan Yates <jordan@embeint.com>