Remove requirement for rate-monotonic scheduling

[ArcEye]

Issue by zultron Thu Oct 16 15:12:55 2014 Originally opened as https://github.com/machinekit/machinekit/issues/335

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Because of legacy design decisions, Machinekit requires rate-monotonic scheduling to guarantee coordination between threads. There's a long and informative discussion on the emc-developers list, and mention of the problem in #237.

Issues arising from this requirement:

• Machinekit RT threads can't take advantage of multi-processors
• Machinekit can't easily coordinate with hardware peripherals onto which RT tasks might be offloaded
• Elevated privileges are required even for POSIX threads
• Potentially limits porting Machinekit to RT environments where rate-monotonic scheduling is unavailable

[ArcEye]

Large issue with many comments, please use original at https://github.com/machinekit/machinekit/issues/335