Extract the authority list from config.c

There is a list of default_authorities in src/or/config.c. We want it in a separate file at src/or/auth_dirs.inc.

This should be implemented like the default_fallbacks array, which includes the fallback list from src/or/fallback_dirs.inc.

We will need to implement two branches for backporting:

• a branch on maint-0.2.9 for 0.2.9 and later. It has IPv6 addresses.
• a branch on maint-0.2.5 for 0.2.5. It has no IPv6 addresses.

(Then, after we have moved it into a separate file, we want to automatically generate the file, in a new format. See the rest of the children of #24818 (moved).)