Report error when trying to create a reservation with "nodes=" (empty list)

Without this change, an empty reservation is created
Clarify in log that changing the features on an existing reservation is not allowed
Fix for priority/multifactor2 plugin to not assert when configured with
--enable-debug.
Merge remote-tracking branch 'origin/slurm-2.5'

Conflicts:
	src/plugins/priority/multifactor2/priority_multifactor2.c
Change strstr to slurm_strcasestr