This commit started as removing FIXME comments, but in doing so I
realized that the IP allocation process was using unvalidated user
input. Before de-layering, validation was called twice - once before
init and once after, which the init code depended on.
Fortunately (or not?) we had duplicative checks that caught errors but
with less friendly messages.
This commit calls validation before initializing the rest of the
IP-related fields.
This also re-organizes that code a bit, cleans up error messages and
comments, and adds a test SPECIFICALLY for the errors in those cases.
650f8cfd35