We currently generate a complete, binary dconf database
as part of the install process.

In general, system administrators can change dconf
settings system-wide using a mechanism described here:

https://live.gnome.org/dconf/SystemAdministrators

Unfortunately, because of the way we generate our database,
any administrator overrides will get discarded during
package upgrade, and likewise, our carefully vetted database
will get destroyed on any site administrator updates.

Rather than step on each others toes, this commit changes
GDM to use the same mechanism described above.  This should
make upstream defaults and system defaults coexist much
better.

Note, the mechanism used isn't quite as featureful as we
would really like.  It doesn't support wildcards or changing
whole subtrees at a time.  That means, in theory, if a new
plugin gets added to settings daemon it could be marked active
at the login screen without being vetted first.

To circumvent this, we add a layer of indirection in and some
gross shell scripting to give us the features we need.

Distributions should probably run

/usr/libexec/gdm-update-dconf-db \
	gdm \
	/usr/share/gdm/upstream-settings \
	00-upstream-settings

In there post transaction hooks, but at a minimum need to run
dconf update.