Add a helper script to improve accuracy when benchmarking

Add a helper script to improve accuracy when benchmarking

This script put the CPU in 'performance' mode to avoid distortion in the
results because of CPU frequency changes. It also gives the process
maximum CPU time and I/O priority using nice and ionice respectively.
Finally, it only uses the number of CPUs specifiend in the CPUS
environment variable to run the program.

The scripts is used like this:

	CPUS=2 ./bench.sh some_prog --some-args

This runs "some_prog" passing "--some-args" as arguments, using 2 CPUs.