They now dynamically allocate stdio message buffers rather
than statically allocating them at startup.

onto the trunk.

Major changes:

- Removes the slurmd's use of SysV shared memory.  Instead the slurmd
  communicates with the slurmstepd processes through the slurmstepd's
  new named unix domain socket.  The "stepd_api" is used to talk to the
  slurmstepd (src/slurmd/common/stepd_api.[ch]).

- Reorganizes the src/slurmd subdirectory.

- Rename the "slurmd_step" process "slurmstepd".

Minor change:

- Fix an srun bug that was introduced in the srun forking changes

No real changes in logic.