Print square brackets around the tag value in RADIUS strings

Before, VLAN attributes that had a tag 1 looked like this:

  Tunnel Medium Attribute (65), length: 6, Value: Tag[1]802
  Tunnel Private Group Attribute (81), length: 4, Value: Tag 14

With the Tunnel-Medium-Type attribute (65), it is clear where the tag ends and the value begins. With this patch, the value for a string type (like Tunnel-Private-Group-Id) looks similar:

  Tunnel Private Group Attribute (81), length: 4, Value: Tag[1]4