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
d56f51ea7d