a final few more features in stress-ng

While hoping to get a feature complete stress-ng sooner than later, I found a few more ways to fiendishly stress a system.

Stress-ng 0.01.22 will be landing soon in Ubuntu 14.10 with three more stress mechanisms:

• CPU affinity stressing; this rapidly changes CPU affinity of the stress processes just to keep the scheduling busy wasting effort.
• Timer stressing using the real-time clock; this allows one to generate a large amount of timer interrupts, so it is a useful interrupt saturation test.
• Directory entry thrashing; this creates and deletes a selectable number of zero length files and hence populates and destroys directory entries.

I have also removed the need to use rand() for random number generation for some of the stress tests and re-used a the faster MWC "random" number generator to add in some well known and very simple math operations for CPU stressing.

Stress-ng now has 15 different simple stress mechanisms that exercise CPU, cache, memory, file system, I/O and CPU schedulers.  I could add more tests, but I think this is a large enough set to allow one to thrash a machine and see how well it performs under pressure.