Import NetBSD libpcap

Import NetBSD libpcap

As part of this, we import bpf_filter.c from NetBSD.  Even though that
file is part of the NetBSD kernel, it is also used by userland (as is
clear here).  Our LWIP service has its own bpf_filter.c implementation
but that implementation has certain limits (e.g. on program size) that
are fine for a system service but should not apply to userland.

The libpcap code has a number of blocks guarded by __NetBSD__, but
none of those blocks apply to MINIX 3.  In particular, some of the
alignment logic used for NetBSD may in fact not work in our case.

Change-Id: Ib187e22d627c929e111d5d4a991c3bee3c0154cb