netifyd supports a '-F' filter option in 'bpf' notation to filter
packets from its consideration.

Add support for a uci 'filter' option. eg. filter to exclude SSDP
multicasts from a particularly noisy device:

option filter 'not (udp and dst 239.255.255.250 and dst port 1900 and src 192.168.1.5)'

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>