Improve filtering capabilities: Berkeley Packet Filter

[GyulyVGC]

This PR replaces the old, limited filters with a new single filter allowing to specify an arbitrarily complex Berkeley Packet Filter program.

This filter is applied directly to the PCAP source, hence it does the filtering at lower level without the need for Sniffnet itself to match packets against filters.

This improves performance with the drawback of losing information about the number of filtered-out packets.
In fact, PCAP doesn't offer a reliable API to see the filtered out-packets; from pcap_stat manual page:

The statistics do not behave the same way on all platforms. ps_recv might count packets whether they passed any filter set with pcap_setfilter or not, or it might count only packets that pass the filter. It also might, or might not, count packets dropped because there was no room in the operating system's buffer when they arrived.

This PR also considerably renews and simplifies the app’s initial page.

bpf.mp4

Fixes #810.

[GyulyVGC]

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