[3.0.0 prep] New pcap_* bindings

[cicku]

• pcap_init
• pcap_dump_ftell
• pcap_dump_ftell64
• pcap_dump_file
• pcap_snapshot

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pcap_init fixes #383 and supersedes #384.

Must be merged after #400, and a rebase will be done then.

[cicku]

Ready:

Reason of additions:

• pcap_init: Non-ASCII handling
• pcap_dump_ftell64/ftell: Size-based file rotation needs
• pcap_snapshot: For Capture<Offline> the snapshot length comes from the file header and is otherwise unknowable

I'm open to drop pcap_dump_file if you feel like certain low level functions are not needed. Instead one can always flush() and offset().