First step towards fixing srun signalling when using --attach --join.

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.