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

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

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