Update to 2.1

• Performance: added support for filtering incoming packets at the kernel level using BPF
• Various minor fixes and improvements.

PR: 250016
Submitted by: Oleg Derevenetz <oleg.derevenetz@gmail.com> (maintainer)
Approved by: arrowd (mentor)
Relnotes: https://github.com/oleg-derevenetz/bwping/releases/tag/RELEASE_2.1
Differential Revision: https://reviews.freebsd.org/D26641