net/iperf2: Add version 2.0.13

As discussed on pksrc-users@, this is the currently-maintained, more
or less, continuation fork of an older fork (in net/iperf) that is not
maintained.  It's net/iperf2 to avoid misleading users looking for
"iperf" into thinking that it is the normal approach.

Relative to net/iperf, this update drops a patch deleting upstream's
ad hoc definition of bool, because it fails to build on NetBSD with
that modification.

Iperf is a tool for measuring TCP and UDP bandwidth performance.
It reports bandwidth, delay jitter, datagram loss. It also supports
IPv6 and IP Multicast.

This version is a continuation fork of an older continuation fork of
the original code.  The original code and the first continuation fork
are no longer maintained.  See also net/iperf3.

(gdt)