libiperf catchup

# Context

* Version of iperf3: any

* Hardware: all

* Operating system (and distribution, if any): all

# Enhancement Request

A number of enhancements have been made to iperf3 that caused the `iperf_test` and `iperf_settings` to grow some new members corresponding to new configuration parameters. In many cases, the libiperf API was not extended to include getters and setters for these parameters, with the result that programs using libiperf don't have access to all of the test parameters that can be specified from the command-line.

The libiperf API should catch up to the reality of these structure definitions.