A bit of scaffolding for handling the lines from the client telling us
about its commits.

This lets us model the have/want lines from the client.

Let's start with baby steps, by letting the server figure out what type
of request the user wants.

This includes some include guards for internal headers, which apparently
we've only ever included once up to now.

Fixed miscellaneous documentation errors.

We go through the different backends in order, so it's not an error if
at least one of the backends has the data we want.

We have too many places where we repeat free code, so when adding the
new free to the generic code, it didn't take for the local transport.

While there, fix a C99-ism that sneaked through.