With the new stdio engine, it no longer makes sense to associate the task
stdio filenames with the srun info structure.  Instead for each task we
expand the filename format string and stick it in the slurmd_task_info_t
structure.

Add new env var SLURM_TASK_PID.

filename parameters.

Previous logic would deadlock on fork if log_init() was followed by 
log_fini() since log_fini destroys the mutex that atfork works with.

 needed thanks to slurmd_step functionality.

still open.  Each stdout or stderr that closes will trigger an
eof message (header.length will be 0), and _server_read will
decrement the number of remote writers.

is a pointer.

This merge is a rewrite of slurm's task standard I/O handling.
srun will now have just a single TCP stream to each node of a job-step.
srun and slurmd comminicate over the TCP stream using a simple messaging
protocol.

Both srun and slurmd now use a completely event driven I/O engine based on
the eio.c event I/O engine.

Be warned that the new I/O support in this merge is not quite complete.  The
core I/O protocol is functional, but it is still missing reattach support
and it needs extensive testing and debugging.

   TaskProlog, TaskEpilog, TaskPlugin; srun --task-prolog, --task-epilog).