dhcp-relay: Filtering existing option 82

[tohojo]

When adding an option 82 to the relayed DHCP packets, we should filter any
existing options as well.

[yoelcaspersen]

As clients are untrusted by definition, client originated DHCP packets containing Option 82 should be discarded - from RFC3046 section 2:

Relay agents receiving a DHCP packet from an untrusted circuit with giaddr set to zero (indicating that they are the first-hop router) but with a Relay Agent Information option already present in the packet SHALL discard the packet and increment an error count.

[sachintiptur]

@tohojo @yoelcaspersen is there any task that i can pick it up?