Allow Fallback Directories with no DirPort

In legacy/trac#12538 (moved), we made almost every relay support begindir-style directory fetches. In legacy/trac#18483 (moved), we made clients never use DirPorts. Both of these features are scheduled for 0.2.8.

Eventually, we can allow fallback directories with no DirPort. (Perhaps this works already with a zero DirPort?) If we implement legacy/trac#7798 (moved), we might want this feature, but only for directory guards in the state file.

If we want to include fallback directories with no DirPort in the default fallback list, we will need to make some some changes to the fallback directory checks in scripts/maint/updateFallbackDirs.py. In particular, stem can't do begindir, so it will be harder to verify these fallback directories have connectivity and support begindir. So maybe we should require a DirPort for fallback directories for the moment.