Allow programmatic shutdown of iperf3 server

[dongfengweixiao]

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

master

• Issues fixed (if any):

none

• Brief description of code changes (suitable for use as a commit message):

when iperf3 is running on server mode, and it wait client connect, when timeout>0, the process is in a blocked state, so cannot close server programmatic.

when timeout >= 0, server side can close server by:

iperf_set_test_state(test, IPERF_DONE);
iperf_set_test_one_off(test, 1);