RFC: Name the first release from a branch N.1.0 instead of N.0.0

[tstellar]

@h-vetinari

For 11.0, the SOVERSION changed from 11 → 11.1, but presumably people would want to keep SOVERSION = N also in the new scheme, meaning that we’d have to go from SOVERSION = N for N.1 to SOVERSION = N.2(?) for N.2. That’s annoyingly inconsistent, but perhaps acceptable given how rare such unintentional ABI breaks should be.

I think if we implement this proposal, we should also add the minor version to the SOVERSION. N.0.0 (trunk) would have a different ABI than N.1.0 (release branch).