config: Restore old semantics for [settings] and [default]

* bin/bugz: Check for config-inherited values.
* bugz/configfile.py: Force all sections inherit from [default], do not look
* for "type" option.
* conf/pybugz.conf: Reflect changes.
config: Allow the option 'inherit' in configuration

* bin/bugz: Bugfix.
* bugz/configfile.py: Calculate the inheritance hierarchy.
* conf/pybugz.conf: Document 'inherit' option.