Add fifo option to FileCapture

[tranzmatt]

This allows FileCapture to read from a FIFO, which can be useful for parsing the kismet pcapng stream.

mkfifo /tmp/sharkfin || true
wireshark -k -i /tmp/sharkfin &
curl -N -u user:password http://localhost:2501/pcap/all_packets.pcapng > /tmp/sharkfin &

[KimiNewt]

There appear to be a bunch of unrelated things in this PR, please only contain the feature in the PR title.