Fix for used_cpu_run_secs bad calcuation

Add argument to priority plugin's priority_p_reconfig function to note
when the association and QOS used_cpu_run_secs field has been reset.
Without this flag, we remove time on "scontrol setdebug" or
"scontrol setdebugflag" that can result in used_cpu_run_secs
going negative or otherwise get bad values.
Correction to logic added in commit 6d793189
bug 423