gkeys/config.py: Subclass ConfigParser to be able to set defaults in the config

Allow some base defaults to be set in the [base] section of the config.
Reset our defaults from those changes.
Resave the new defaults back to the configparser for interpolation when retrieving the values.
gkeys/base.py: Make the category/seedfile choices dynamic
gkeys/lib.py: fix a missed mode setting for an ensure_dirs() call
gkeys: Use the new category handling system
gkeys/actions.py: Add a listcats action
gkeys/actions.py: Improve category error message
gkeys/actions.py: Fix listseed action for the new category/seedfile handling
gkeys/etc/gkeys.conf.sample: Update for new category/seedfile handling
gkeys: Use -C, --category exclusivley for consistency

No longer use -S, --seedfile for seed file operations.
With the new dynamic category handling, it was redundant.