Disable default fallback directories when other directories are set

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 90f6071d in 0.2.4.7-alpha. Patch by "teor".