Build with -Wextra and modernise the capture device setup

[git-hulk]

• hashtable compared strlen's size_t against an int in three places,
  which is what -Wextra was reporting, and read a uint32_t straight
  out of the key with a cast. That load is undefined for an unaligned
  key and faults outright on a strict-alignment target; memcpy says
  the same thing and compiles to the same instruction.

  • sniffer_online called pcap_lookupnet purely to bail out when it
    failed: neither the network nor the mask it returns is ever used.
    That made a capture on an interface with no IPv4 address fail for no
    reason. Dropped.

  • Replace pcap_lookupdev, which is deprecated and hands back a shared
    buffer, with pcap_findalldevs. The fallback now also skips loopback
    unless it is the only device there is. Reaching this path needs
    root, since the uid check runs first, so it is covered by inspection
    and by exercising pcap_findalldevs directly rather than by a test.

  • Turn on -Wextra now that the tree is clean under it.

🤖 Generated with Claude Code