runtests: catch EPROTONOSUPPORT in checkportisavailable...
runtests: catch EPROTONOSUPPORT in checkportisavailable
This is a follow-up of "runtests: check ports on IPv6 address". On some
platforms, "socket.AF_INET6" exists while that does not necessarily mean the
platform support IPv6 - when initializing a socket using "socket.socket", it
could fail with EPROTONOSUPPORT. So treat that as "Port unavailable".