Pulling back some minor refactoring and bug fixes from master.
- Fixes: https://github.com/XQuartz/XQuartz/issues/6
- Fixes: https://github.com/XQuartz/XQuartz/issues/32
- Fixes: https://github.com/XQuartz/XQuartz/issues/40
- Fixes: https://github.com/XQuartz/XQuartz/issues/48
- Fixes: https://github.com/XQuartz/XQuartz/issues/56
Also fixes a regression that caused a crash during shutdown when freeing fbconfigs.