This needs: * a proposal and a design * a patch to dir-spec.txt * a patch to the tor man page * a tor code patch * an updateFallbackDirs.py code patch * a wiki update to [[doc/UpdatingFallbackDirectoryMirrors]] Here's a quick sketch of a design: 1. Relay operators set `OfferFallbackDir 1` to offer their relay as a potential FallbackDir. 2. Relays with this option set put `offer-fallback-dir` in their descriptors 3. updateFallbackDirs.py loads relay fingerprints with `offer-fallback-dir`, and from the legacy offer list 4. updateFallbackDirs.py does stability checks, and generates the fallback list