This option simulates payload in iperf2, which is just repetitive pattern
(ASCII '0..9' repeating), as opposed to iperf3 where payload is random.
It might help in testing and reveal problems in networking gear with hardware
compression (including WiFi access points), where iperf2 and iperf3 perform
differently, just based on payload entropy.
* EXIT_SUCCESS and EXIT_FAILURE are defined in stdlib.h, not sysexits.h
so no need to include the latter.
* No need to include pthread.h.
* Remove the no longer used delay() function.
This also removes the reliance on nanosleep().
* Remove get_tcp_windowsize() and set_tcp_windowsize() which are unused.
These days, iperf uses get/setsockopt() with SO_SNDBUF SO_RCVBUF directly.
not including it.
To fix this required us to change config.h to iperf_config.h (to
avoid potential filename collisions with this generic name). Then
iperf.h could include this.
Adjust the existing header file inclusions to track this, and also
canonicalize their inclusion to be at the top of *.c files.
