doesn't keep track of anything yet.

Renames some sview variables for better clarity and put some node
  fields into alphabetic order

This commit adds support for free_mem as part of the sinfo
command using the key 'e' ('f' and 'F' are already taken).

This commit adds information about the free memory to the
output created by slurm_sprint_node_table(). The unit of
FreeMem is MiB like RealMemory.

This commit adds a new field free_mem to the node_info struct
and augments the 15.08 RPC protocol to exchange this information
as part of the ping and registration status messages. The
free memory is taken from the sysinfo() information on systems
that support this function. Overall, the implementation sticks
to the way cpu_load (the 5 min load average) is handled.