Add benchmarks/uperf

Unified Performance Tool (or uperf for short) is a network
performance tool that supports modelling and replay of various
networking patterns. It represents a new kind of benchmarking tool
(like filebench) where instead of running a fixed benchmark or
workload, a description (or model) of the workload is provided and
the tool generates the load according to the model.

PR: 226649
Submitted by: 0mp@FreeBSD.org
Requested by: gnn