Make slurmstepd no longer call fatal() on a client read error.

Fixes test13.1 failure.

globally blocked in slurmstepd.  Also fix bug in the test itself.

It previously just excluded specific nodes with completing jobs and ran 
jobs on larger bglblocks than required.

especially after and --attach

Fixes fallout from the srun message handler fork() by passing the
slurm_step_layout_t data from the message process to the main srun
process. (The MPIR debugger proctable stuff should probably be handled
in the same place, rather than having four seperate messages of its own,
PIPE_MPIR_*).

Also rewrote par_thr to use a switch statement rather than a long
string of else-if statements.

fix