make cross-platform `EADDRINUSE` check

[aybabtme]

This function only works on unix, and not on windows. Make it cross platform.

humanlog/cmd/humanlog/localhost.go

Lines 35 to 52 in 4aff753

	func isEADDRINUSE(err error) bool {
	nerr, ok := err.(*net.OpError)
	if !ok {
	return false
	}
	nserr, ok := nerr.Err.(*os.SyscallError)
	if !ok {
	return false
	}
	if nserr.Syscall != "bind" {
	return false
	}
	nserrno, ok := nserr.Err.(syscall.Errno)
	if !ok {
	return false
	}
	return nserrno == unix.EADDRINUSE
	}

We want to know when we're trying to use an already used port, because that's how we find who's the localhost leader in a multi-process scenario (e.g. if multiple humanlog processes are running concurrently). The leader is whoever has the port.

• enable CI (commented out in b7110ad)

[aybabtme]

solved by #121