Reintroduce compatibility with old 0.x clients

Currently the command that gets launched on the receiver side is installed as ostree-receive, as it was before the transition to the new reverse-pull-over-http protocol.

This means that once the server is upgraded, older clients break badly since the receiver side no longer understands their protocol.

To address this issue and retain compatibility with old clients:

1. install the new server side code with an unambiguous name, ostree-receive-1
2. make the new client do a lookup to see if ostree-receive-1 is availabile before falling back to the ambiguous ostree-receive name
3. revive the old 0.x code and install it as ostree-receive-0
4. add a ostree-receive script to exec the right version depending on the invocation parameters (the --repo parameter is only used by the old code)

Apertis: T9905

Upstream: https://github.com/dbnicholson/ostree-push/pull/12