Changes from Pavel Kankovsky to reset the kernel filter in

Changes from Pavel Kankovsky to reset the kernel filter in
"pcap_setfilter()" if we're not using a kernel filter, in case a
previous call to "pcap_setfilter()" had succeeded in adding a kernel
filter, as if we're doing userland filtering we need to get rid of all
kernel filters that might discard packets that'd pass the userland
filter.