Add RCUTILS_NO_PROCESS option to disable subprocess suppport #813

• Hardware description: YRC1000
• RTOS: VxWorks
• Installation type: custom build for creating MotoPlus app
• Version or commit hash: Kilted

I'm working on getting ROS2 Kilted running on Yaskawa robot controllers using micro-ros. But ros2/rcutils@937efe2 wait.h, fork, waitpid, and other process components were added to rcutils. The kilted micro-ros patch didn't touch this. Not all RTOS support such functionality. I think that it would be good if there was another option like these which disables creating subprocesses.

In the short term, I created a patch specifically for Yaskawa controllers that achieves this (see here), but I think that should make its way to the Kilted and future micro-ros patches.