Have get_parent_directory() handle "/foo" and "/" correctly.

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.