Allow --zerocopy and --skip-rx-copy when only server supports, fix -P value verification

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master 3.21+

• Issues fixed (if any): zerocopy os check should wait for sever response when in reverse mode #2044

• Brief description of code changes (suitable for use as a commit message):

Allow --zerocopy and --skip-rx-copy even when one of the sides OS, client or server, not support them, as they may be supported by the other side. When not supported, allow zerocopy only when not sending and skip-rx-copy only when not receiving.

(UPDATE note: deleted the -P related changes and created a separate PR for them.)

[bmah888]

Thanks for the pull request. I'm trying to test it, but it looks like this is a fix (with respect to --zerocopy and --skip-rx-copy for a problem that does not occur on Linux, FreeBSD, or macOS (the three main development problems). If this is only an issues (a platfom that we don't officially support) this is going to be a little hard for our team to test. Haven't quiet figured out how to deal with this yet.

[davidBar-On]

Haven't quiet figured out how to deal with this yet.

I tested it by creating a version with HAVE_MSG_TRUNC set to zero, and then run it once as a client and once as a server (when the other side is the version with HAVE_MSG_TRUNC set to 1). I believe this is sufficient as each of these options affect either sending or sides but not both.