Allow udisks2 and upower backends to be disabled with environment variables
udisks2 and upower are kind of shaky and noisy if enabled but masked.
I have masked e. g. upower because I don't like things hanging, but Solid still tries to use it so I get stuff like this every now and then: virtual QStringList Solid::Backends::UPower::UPowerManager::allDevices() error: "org.freedesktop.systemd1.UnitMasked"