Allow multiple executions of same command with runexec

[PhilippWendler]

To increase confidence in time results, it may sometimes be useful to run the command multiple times, and report all results, as well as mean/median/standard deviation etc. This could be implemented in runexec, similarly to what dumbbench does.

[PhilippWendler]

The Execution-Time Measurement Protocol is an approach that makes use of multiple executions of the same command for increasing the measurement precision.