The parent of "/foo" is "/"; and "/" is its own parent.
This would cause Tor to fail if you tried to have a PF_UNIX control
socket in the root directory. That would be a stupid thing to do
for other reasons, but there's no reason to fail like _this_.
Bug found by Esteban Manchado Velázquez. Fix for bug 5089; bugfix on
Tor 0.2.2.26-beta. Unit test included.