Server reset errno when starting new test

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master 3.21+

• Issues fixed (if any): none

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

Reset errno when starting new test. Currently, if test 1 is terminated when errno is set and then test 2 is terminated without setting errno, the server send to the client errno from test 1.

The issue can reproduced by e.g. iperf3 -s --server-max-duration 10 and then:

1. First test: iperf3 -c ... -t 5 and terminating the client before the test ends (e.g. using ^C). Server's errno is set to Connection reset by peer.
2. Second test: iperf3 -c ... -t 20. The server's error about exceeding the maximum test duration is returned with errno from the first test.