Add way to specify snapshot length and buffer size when using pcap

When using pcap and a ring buffer for receiving packets, it can be useful to specify the snapshot length and the buffer size when receiving packets. For example, if you know that you'll only want to be processing small packets, then it could be worth setting the snapshot length to be a small value. This has a benefit on systems where libpcap uses TPACKET_V2, where the number of packets that can fit in the ring buffer depends on the size of the ring buffer and the snapshot length; smaller snapshot lengths means more packets can fit there.

Similarly, if you know you might want to handle large packets, then having an option to increase the buffer size could be beneficial.

Locate the pcap receive path and its ring-buffer setup, then read how libpcap and TPACKET_V2 currently determine snapshot length and buffer size. Define where these two settings should be exposed and verify that configured values affect packet capture without breaking existing defaults.