Network seed derivation doesn't take ship parameter

[Fang-]

We currently derive the networking seed according to the text of the spec, but not to the figure contained therein.

To clarify: the visual derivation tree in the Urbit Wallet spec shows the ship being included in the salt for networking seed derivation. The text used to describe this, but erroneous editing in urbit/fora-posts#5 mangled the text, excluding (indirect) mention of this.

This matters, because in practice, a single master ticket may own multiple ships, and we will want unique networking keys for all of those. As it stands, owning multiple ships in the same Urbit Wallet will end up with them all using identical networking keys.

Adding this functionality in is desirable, but may break backwards compatibility, depending on how we go about implementing it.