Add a torrc option and descriptor line to opt-in as a FallbackDir

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