When attaching a "bpf_program" to a "pcap_t" to use as a userland

When attaching a "bpf_program" to a "pcap_t" to use as a userland
filter, always attach a copy, as "pcap-linux.c" does; that way, after a
program uses "pcap_setfilter()", it can safely use "pcap_freecode()" to
free up the BPF instructions allocated by "pcap_compile()".  Also,
always free it up when the "pcap_t" is closed.

Get rid of the "pcap_t *" argument to "pcap_freecode()", as it's not
necessary.

Document "pcap_freecode()", for the benefit of programs that might
repeatedly compile filter programs and attach them, so that they can
free them up after attaching them and avoid leaking memory for them.