Completely remove the slurm_spawn() API call and all supporting code.

slurm_spawn() is replaced by slurm_step_launch() with a "user_managed_io"
flag.

Additionally, there is a new slurm_step_ctx_daemon_per_node_hack() API
function to support poe's behaviour of grabbing a step context for the
application, but then launching pmd under the context (one per node) rather
than the actual application tasks.

Merge of temporary working branch:

  -r9459:9500 https://eris.llnl.gov/svn/slurm/branches/spawn_consolidation