Support for -D got broken in the 2.0.11 release by the upstream commit
218d8c667944 ("first pass L2 mode w/UDP checks, v4 only"). After that
commit clients were still able to connect but no traffic was passed.
It was reported and is fixed now in the upstream git repository.
Backport two patches to fix this. The first one is just a requirement
for the later to apply. The second one is the real fix and it needed
only a small adjustment to apply without backporing the commit
10887b59c7e7 ("fix --txstart-time report messages").
Fixes: 457e6d5a27 ("iperf: bump to 2.0.12")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
