travis: run "make distcheck" with the appropriate configure arguments

Otherwise, we just use the default ones...
bsd: fix clang warning about alignment problem with sockaddr

While this does not seem to be extensively documented, when casting a
`sockaddr *` to something else, like `sockaddr_dl *`, it is believed
that initially, it was a `sockaddr_dl *`, so there is no alignment
problem in this case. We use a macro to cast to `void *` to avoid the
warning.
bsd: ensure we also have space for the BPF header
bsd: ensure proper alignment of BPF header

This is required as per bpf(4).
bsd: ensure we also have space for the padding after BPF header