Fix handling of counter resets

Fix handling of counter resets

There is no reason for or need to change max_hours, ir->nsteps or
step_rel when doing a counter reset.

This makes clear that the behaviour for the combination

mdrun -maxh t -resetstep n

matches the documentation of -maxh.

Updated the API for walltime_acccounting and its usage, because
elapsed time is an insufficiently clear context. Changed the names of
the start and stop functions so that no callers can silently rely on
semantics that have changed.

Avoided variables such as elapsed_time and max_hours, which were
insufficiently precisely worded.

Refs #1781

Change-Id: I16c96985f43a7b4ac75b94f378da3d05914d6986