sched/backfill - Prevent invalid memory reference

if bf_continue option is configured and slurm is reconfigured during
one of the sleep cycles, then the backfill scheduler will reference
an invalid partition pointer.
Documentation correction for JobAcctGatherFrequency and acctg-freq
Update man pages information about acct-freq and JobAcctGatherFrequency
to reflect only the latest supported format.
x is past in as something like ("print %s", buf) that can not be compared
to NULL.

At least if you try the compiler gives this warning

warning: left-hand operand of comma expression has no effect [-Wunused-value]

In 13.12 this is treated as an error
Bring back original functionality using DEBUG
Merge remote-tracking branch 'origin/slurm-2.6'