Clear some pthread values after join

This will prevent possible confusion for the backup controller
when it switches from primary back to backup modes since those
pthread IDs are no longer value. Note the thred_id_rpc could be
used by the backup controller after returning to backup mode
Enable faster termination of backup controller on SIGTERM

Previously there was a sleep(5) during which the backup controller
was non responsive during its startup mode or returning from primary
mode.
Merge branch 'slurm-2.6' of https://github.com/SchedMD/slurm into slurm-2.6