Fix compatibility bug with newest version of libpcap

You're not supposed to call pcap_compile before calling pcap_activate.
This worked on older versions of pcap, but the newest version
has a check that throws an error when you try to do this.

I split the method into SetFilter and ApplyFilter, so that places
that it's used before the capture is activated can use SetFilter,
and places used for live updating of the filter can use SetFilter
and ApplyFilter.