Mostly this change consists of adding FreeBSD-specific code to handle
this feature. The concepts and system calls are very similar to what's
already done for Linux. One difference is that on FreeBSD, the CPU
affinity mask is saved before -A processing and restored afterwards.
This causes a slight change to the function signatures for
iperf_setaffinity() and iperf_clearaffinity() (these functions
however are not documented as a part of the libiperf3 API).
Slightly improve some of the documentation for the -A command line
option, to hopefully stave off some of the questions about this
feature.
Mostly based on a submitted patch.
Issue: 128 (better error message for CPU affinity failure)
Submitted by: Susant Sahani <ssahani@redhat.com>
