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

the PATH.

instead of #define in the file.

A connection with a slurmstepd is established with the stepd_connect() call,
and then all of the following stepd_* calls take the file descriptor
that was returned by stepd_connect.