sem-scalebench: Misc minor updates

- Rename cpus to threads where applicable:
  It is now possible that multiple threads run intentionally on
  the same core
- Allow to set the maximum cpu number explicitely
- Detect if - due to rounding - the same cpu count is specified
  twice, and then skip it.

If you want to run the same test multiple times intentionally,
then use -f -c 1,2,1,2,1,2

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>