Easier multi-arch image building

• build a multi-arch image for both arm64 and amd64
• go: use narrower UDPListener interface
• Handle random port correctly: report assigned port
• correct the ounit url
• go: remove hardcoded GOARCH to support multiarch builds
• transport: fix off-by-one in Unix socket code
• transport: max socket length is different on Linux
• transport: use the path shortener for all Unix domain sockets
• transport: move the path shortening functions to unix_unix.go
• transport: work around Unix socket path lengths on Darwin
• Add guillaumerose to maintainer list
• Upgrade linuxkit/virtsock vendoring
• Isolate vpnkit http server in a separate package