fix -k, --fwmark with setuid fping executable

fix -k, --fwmark with setuid fping executable

* Setting the SO_MARK socket option requires root privileges (or
  CAP_NET_ADMIN), thus temporarily elevate privileges for this
  operation, if possible, by using the p_setsockopt() function.
  This allows to use -k, --fwmark with setuid fping.
* Adjust README.md since setuid now works for -k, --fwmark.
* Remove "sudo" from testing -k since it is no longer needed.
* Test failure of setting firewall mark without privileges or
  capabilities for both IPv4 and IPv6.