* feat: Add a mechanism to disable checks for SCTP (--without-sctp).
The use case for this is building a static iperf3 binary on CentOS 7,
on a system with SCTP installed (but it has no static SCTP libraries).
In that case we need to disable SCTP detection to prevent a linking
error at runtime.
While here, s/iperf /iperf3 / in a couple of help strings.
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