"Soft realtime" mode for userspace models

This code, together with the userspace gpstime and mbuf compile-time options, lets models run in a normal Linux host or container as ordinary userspace programs. The models cannot be used to drive ADC/DAC hardware, but can be useful for simulation or testing purposes.

Instead of waiting at the end of each cycle, models wait only at the end of each 16 Hz DAQ block. The rest of the time, they run as fast as they can while staying in sync with each other. As long as they finish each 16 Hz block with several milliseconds to spare, they can absorb the normal OS scheduling jitter.

Enable this by adding softrt=1 to the CDS parameters.