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* 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.