launch/poe - Fix for hostlist file support with repeated host names.

Without this change, it appears that POE ignores the -procs argument
resulting in a job step request with multiple host names, but only
one ntask required
Update META for v2.5.7 tag
Start NEWS for v2.5.8
priority/multifactor2 - Prevent possible divide by zero.

Andy Wettstein (University of Chicago) reported privately to me that slurmctld
2.5.4 crashed after he enabled the priority/multifactor2 plugin due to a
division by zero error.

I was able to reproduce the crash by creating an account hierarchy where all
the accounts and users had zero shares.
See bug 315
Merge branch 'slurm-2.5'

Conflicts:
	META
	NEWS
	src/plugins/priority/multifactor2/priority_multifactor2.c
Update META for v2.6.0-rc1 tag
Change spaces to tabs in some new work
Expand build notes description