Better command line parsing wrt benchmark execution

[ceresek]

The current command line parsing code does not warn when multiple execution policies (--repetitions <count> and --run-seconds <seconds> and --operation-run-seconds <seconds>) are specified on the command line, and simply uses the last (?) one for everything.

Ideally, we should use the policies in a logical or, so that if any tells the harness to stop, it stops. (In practice, we needed a combination of max iteration count and max time, which could be done by a plugin, but having logical or could be better ?).