Avoid using pointer arithmetic in the BPF program, so that it requires
only CAP_BPF and not CAP_PERFMON. In this context "pointer arithmetic"
means adding a variable value to a packet pointer. This means that the
program no longer tries to parse variable-size headers (IPv4 options,
IPv6 extension headers). Those were already not supported before. It
also doesn't parse VLAN tags, but there should be no need for that. If
we use fixed offset, we can avoid using the parsing helpers from
libxdp.
Alpine is especially interesting because it uses musl as libc.
The build does not yet succeed. There are several issues that
need to be fixed.
However, it will be simpler to fix things, if we have tests
in place -- even if at the moment they are known to be broken.
See-also: https://git.alpinelinux.org/aports/tree/community/networkmanager?h=master
