Given that the netperf project hasn't rolled a release in quite some time,
switch to snapshots directly obtained from the github repository (currently
at version 2.7.1). Generally, there aren't that many changes since the 2.7.0
release version of netperf, the most noteworthy changes are in the area of
FreeBSD support, though:

• FreeBSD-specific code for letting netperf/netserver bind to CPUs, e. g. via the global "-T" option, has been added.
• The nettest_bsd back-end now can take advantage of aio(4) via the newly added test-specific "-a" and "-A" options, which in turn allows zero-copy send and receive with e. g. cxgbe(4). Note that this requires the netperf port to be built with its default-on OMNI option disabled, as otherwise the nettest_omni back-end and the migration to it from legacy back-ends (also the upstream default since netperf 2.5.0) will be used instead.

Apart from that, some test scripts missing in the published releases but
referenced by test scripts already part of e. g. netperf 2.7.0 are now
included.