Allow for failable backend initialization.

After instantiating an FCmdrServiceBackend, call g_initable_init() on
it if it implements GInitable.
FCmdrService: Remove unused variable.
Add a "DConf" D-Bus interface.

The interface thus far has a WriteProfile(filename) method, which writes
a dconf profile tailored for the caller to the given filename.
Add login shell scripts to set DCONF_PROFILE.

These scripts tell fcmdr-service to write an updated dconf profile
for the current user, and if the D-Bus call is successful sets the
DCONF_PROFILE environment variable.

This lockdown method can be trivially circumvented by a user, but
it's the only mechanism dconf currently offers for establishing a
user-specific profile.