Quickstart admin guide: describe file permission better

Directories created for executables, libraries, etc. have permissions
based upon the umask, which may not be desired.
Validate permissions of key directories at slurmctld startup

Report anything that is world writable.
Corrections to SUG13 agenda
Fix some possibly NULL pointers reported by CLANG

None of these have ever occurred, but these changes will harden Slurm
bug 403
Merge remote-tracking branch 'origin/slurm-2.6'