Unit tests for the 10 valid combinations of set/NULL config options
DirAuthorities, AlternateBridgeAuthority, AlternateDirAuthority,
and FallbackDir.
Add assertion in consider_adding_dir_servers() for checks in
validate_dir_servers():
"You cannot set both DirAuthority and Alternate*Authority."
Only add the default fallback directories when the DirAuthorities,
AlternateDirAuthority, and FallbackDir directory config options
are set to their defaults.
The default fallback directory list is currently empty, this fix will
only change tor's behaviour when it has default fallback directories.
Fixes bug 15642; bugfix on 90f6071d8d in 0.2.4.7-alpha. Patch by "teor".
