main/iperf, main/iperf3: Fix iperf/iperf3 conflict, fix openrc script

First MR. Please let me know of any changes to be made :).

• iperf and iperf3 can coexist without special configuration
• Fix issue with PID subdir not existing (use /run/iperf{,3}.pid instead)
• Fix issue with iperf3's init script tries to run /usr/bin/iperf
• Rename iperf3's init.d script and conf.d file to match the binary and package name
• Document correct default port for iperf3