Enhancement per issue 1311 - use same socket for both Bidir directions

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  3.11

• Issues fixed (if any):
  Possibility to use the same port pair under bidirectional mode #1311

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

Suggested enhancement to allow bidir to use the same port in both directions. The approach is using the same socket for both directions. A new optional argument for the --bidir option is added for that: --bidir=1 (the "1" is for one socket/port).

[swg0101]

Note that this doesn't apply on 3.11, but it does on master:

patching file src/iperf.h
patching file src/iperf_api.c
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patching file src/iperf_api.h
Hunk #1 succeeded at 318 (offset -4 lines).
patching file src/iperf_client_api.c
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patching file src/iperf_locale.c
patching file src/iperf_server_api.c