radv: add some infrastructure for fresh forks for each secure compile

In the following commits we want to be able to fork an existing lightweight
fork created at device creation time. In order for the user facing process
to communicate with this new fresh fork we create some members here to hold
FIFO file descriptors and a unique id.

Here we also add a new fork enum that we use to tell the lightweight
process to create a fresh fork.

For more information on why we create a fresh fork see the following
commits.

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