add front_end nodes to node cache, we can now send messages from slurmctld to slurmd on front end nodes

Add front end event triggers and some ping logic (handles not responding, but ping message not sent)

this is only the infrastructure and is untested, but 500+ lines, so
I'll check it in now

add ability for scontrol to show specific front-end node data

the code does not work properly, by i don't want to lose track
of these mods while debugging it

Add src/slurmctld/front_end.[ch] with logic to build slurmctld data structure with front-end info based
upon the contents of slurm.conf