scontrol: warn user that base node state can not be changed on Cray

The base node state (UP, DOWN, ALLOCATED, ...) is handled by ALPS and inferred
from reading the output of ALPS inventory requests.

To avoid inconsistencies, it is not possible for a user to alter this node state.
This patch adds a warning to scontrol if a user wants to change node state through
slurm:

palu> scontrol update NodeName=nid00171 State=DOWN
State=DOWN can not be changed through slurm: use native Cray tools such as e.g. xtprocadmin(8)

The 'meta' states such as DRAIN can still be changed.