Add srun --accel-bind option

Add srun --accel-bind option to control how tasks are bound to GPUs and NIC
    Generic RESources (GRES).
Based in part upon work by Matthieu Ospici (ATOS).
gres/nic plugin modified to set OMPI_MCA_btl_openib_if_include environment
    variable based upon allocated devices (usable with OpenMPI and Melanox).
Reset GRES env vars after task affinity set