Clean up the declaration of the packet-filtering routines.

Clean up the declaration of the packet-filtering routines.

If net/bpf.h declares bpf_filter() one way and libpcap defines it
another way, even pcap-bpf.c needs a declaration that matches how
libpcap defines it, not how net/bpf.h (mistakenly) declares it.
("Mistakenly" because it should *not* be declaring the kernel's version
of bpf_filter() unless it's being used in a *kernel* build; other *BSDs,
and macOS, declare it only in kernel builds by testing for a #define
such as KERNEL or KERNEL_PRIVATE, but NetBSD doesn't - it *should*, but
it doesn't.)

So we rename the internal-to-pcap filtering routine as pcap_filter(),
which is not exported from libpcap, and have bpf_filter() be a wrapper
around pcap_filter() that is exported.

Use pcap_filter(), rather than bpf_filter(), for all filtering inside
libpcap (except for filtering that uses bpf_filter_with_aux_data(),
which we rename pcap_filter_with_aux_data()).

Do the same for bpf_validate(), which is *also* declared in net/bpf.h,
even for non-kernel builds, in NetBSD.

As we're not exporting pcap_filter_with_aux_data(), don't even *declare*
it in a public header; don't declare struct bpf_aux_data in a public
header, either.  That way we can change it without worrying about
breaking APIs or ABIs; we may do that if, for example, we want to
support the "inbound" and "outbound" filters when reading pcapng files,
adding a direction indicator to that structure.

Declare bpf_filter() in pcap/bpf.h even on NetBSD and QNX; pcap-bpf.c
doesn't include pcap/bpf.h (it sets a #define to force pcap/pcap.h not
to include it), so we won't get any collisions if net/bpf.h (which it
does include) declares it.  The only collisions will occur in programs
that include *both* pcap/pcap.h or pcap/bpf.h *and* net/bpf.h, and that
will occur only if net/bpf.h declares bpf_filter() even when building
userland code, and the correct fix for *that* is to fix net/bpf.h not to
declare them in non-kernel builds.