Enforce linux kernel packet validity constraints

[daniel-noland]

Unsurprisingly, etherparse and the linux kernel do not have complete alignment regarding what constitutes a valid packet.

A simple example of this can be found in etherparse::Ipv4HeaderSlice::from_slice failing to check that the source IP is unicast and non-zero (both of which constitute invalid headers).

Given that we are using the Linux kernel as something of a reference implementation, we should make sure that we are enforcing as many of its constraints as possible.

I expect this ticket to stay open for a while, as it is quite a large job with no absolute proof that we are ever done.

That said, I think we should create sub-issues for this ticket on a per-header basis and do our best to take care of each one individually.