Stacked databases and locks are dconf features that allow
management software like Fleet Commander to set system-wide
defaults and overrides centrally for applications.

This patch adds minimal support for the same to the keyfile
backend. We look for a keyfile named 'defaults' and a
lock-list named 'locks'.

Suitable files can be produced from a dconf database with
dconf dump and dconf list-locks, respectively.

The default location for these files is /etc/glib-2.0/settings/.
For test purposes, this can be overwritten with the
GSETTINGS_DEFAULTS_DIR environment variable.

Writes always go to the per-user keyfile.