Add a "setfilter" function pointer to the pcap_t structure, which

Add a "setfilter" function pointer to the pcap_t structure, which
handles setting a filter for a pcap_t.  Have "pcap_setfilter()" call it,
rather than being a per-platform function.  The per-platform functions
don't need to check for an offline capture any more, as they're not
called for an offline capture (and the ones that just call
"install_bpf_program()" don't need to exist at all).