Add O_DIRECT to CTDS

[gervandiepen]

It should be possible to use O_DIRECT (where available) to avoid that the kernel suddenly decides to flush its file cache. That behaviour make real-time systems unpredictable. The APERTIF datawriter starts to loose data packets as soon as the kernel flushes the file cache. Apparently the kernel has less time available to handle the interrupts on the receive side of the data packets, even if doing that on another core.