get_unpack

# File lib/resolv.rb, line 1442
        def get_unpack(template)
          len = 0
          template.each_byte {|byte|
            byte = "%c" % byte
            case byte
            when cc, CC
              len += 1
            when nn
              len += 2
            when NN
              len += 4
            else
              raise StandardError.new("unsupported template: '#{byte.chr}' in '#{template}'")
            end
          }
          raise DecodeError.new("limit exceeded") if @limit < @index + len
          arr = @data.unpack("@#{@index}#{template}")
          @index += len
          return arr
        end