pcap: wake blocked receive via eventfd when daq_instance_interrupt() is called

[ssam18]

pcap_daq_interrupt() currently just sets a volatile bool that the receive loop checks at the top of each iteration. On Linux with TPACKET_V3 and idle/virtual interfaces the in-pcap timeout isn't always honored, so the second pcap_next_ex() call inside pcap_daq_msg_receive() blocks inside poll() and never sees the flag. This leaves a Snort thread parked indefinitely after daq_instance_interrupt() and contributes to the service-stop hang in snort3/snort3#338. This change creates an eventfd in pcap_daq_start() for live captures, has pcap_daq_interrupt() write to it, and replaces the blocking fallback in the receive loop with a poll() over the pcap selectable fd and the eventfd, so an interrupt wakes the thread immediately and the receive returns DAQ_RSTAT_INTERRUPTED. The fast path (packets are already available) is unchanged. Note: eventfd is Linux-specific, happy to switch to a self-pipe if a portable implementation is preferred.

Refs snort3/snort3#338