Use `SKB` or `HW` XDP driver depending on device capabilities

[qdeslandes]

When an XDP program is attached to the kernel, the caller can pick the attachment mode:

• XDP_FLAGS_HW_MODE: offload the program to the NIC. Only select Netronome devices support offloading.
• XDP_FLAGS_DRV_MODE: attach the program in driver mode, requires the NIC to support it.
• XDP_FLAGS_SKB_MODE: generic mode, works regardless of driver support. This negates the speed advantages offered by XDP, as it is called further up the stack, once a socket buffer has been created.

bpfilter will always pick XDP_FLAGS_SKB_MODE as it will always be supported, but that leaves a lot of performance on the table. Instead, bpfilter should probe the NIC to use the most appropriate mode.