driver process should configure RT priority

[gavanderhoorn]

Even with regular (ie: non e-series) controllers, the driver binary should configure itself for RT priority as this will help with more predictable scheduling of controllers.

Especially the ros_control variant of the driver would benefit from this.

To allow users with regular kernels to still use the driver, we should make this configurable, such that failure to configure RT priority for the process could be ignored.