iperf_sctp: disable delayed-SACK by default to fix slow single-flow SCTP throughput

[nshopik]

Single-flow SCTP throughput collapses to a few Mbits/sec on Linux (~300× slower than TCP on the same loopback) because the receiver's delayed-SACK timer (sack_delay=200 ms, sack_freq=2) waits for a second DATA chunk that the blocked sender will not produce — each iperf3 64 KB block lands as one DATA chunk and stalls 200 ms per I/O cycle. Reproducible on both arm64 and x86_64; not architecture-specific.

iperf3 is a throughput benchmark, so disable delayed-SACK by setting SCTP_DELAYED_SACK with sack_delay=0, sack_freq=1
on all three SCTP socket entry points:

• iperf_sctp_listen — endpoint default (FreeBSD-style SCTP_FUTURE_ASSOC)
• iperf_sctp_accept — required on Linux; the listener-side setting does not propagate to accepted associations
• iperf_sctp_connect — covers --reverse where the client becomes the receiver

Also propagate SCTP_NODELAY to both the server's listen and accepted sockets when -N is given, closing a pre-existing asymmetry with iperf_tcp_listen's handling of TCP_NODELAY.

configure gains an AC_CHECK_TYPES probe for struct sctp_sack_info; new code is guarded by HAVE_STRUCT_SCTP_SACK_INFO and ENOPROTOOPT is treated as non-fatal, so platforms without support are silently unaffected.

master and fix performance

SCTP iperf3 -c localhost --sctp -t 10 │ 2.67 Mbits/sec │ 1.42 Gbits/sec

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:

• Issues fixed (if any): slow sctp performance on arm64  #1815

• Brief description of code changes (suitable for use as a commit message): disable delayed-SACK by default to fix slow single-flow SCTP throughput (slow sctp performance on arm64  #1815)

[bmah888]

Thanks for the PR!