Fix potential buffer overflow.

This isn't that big of an issue for 14.03, but 14.11 added more to this
string which could overflow the buffer since sprintf is used instead of
snprintf.  Using xstrfmtcat fixes the issue and is easier to read code.
Merge remote-tracking branch 'origin/slurm-14.03' into slurm-14.11

Conflicts:
	src/slurmctld/job_mgr.c