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

    when job preemption is configured.

    job_submit and job_modify functions as an error and the user request should
    be treated as an error.

    into an array. Patch from Gerrit Renker, CSCS.

   "FairShare" and added more detail to the sshare man page.