This is entirely a cleanup/fixes release. Some fixes here to make the
increasingly-strict toolchain happy, and also some fixes for some
crashers in the GSettings backend and service.
- remove some unused variables (new GCC gives a warning: #640566, another)
- add a mutex to fix multi-threading issue (#640611)
- don't crash if we have no D-Bus
- clean up symbol exports
- fix a crash in the service when using 'reset'
- drop old linker options that were for libtool
