mxm: fixes for compilation warnings.
    
removed set but not used variables and a variable that is unused.

reviewed by miked
cmr=v1.7.4:reviewer=ompi-rm1.7
add alina to the club
Add missing MCA variable enumerator sentinel.

cmr=v1.7.4:reviewer=rhc
Do a better job of reporting bindings - if someone gives a spec that binds us to all processors, then we are effectively unbound and should report it clearly instead of outputting a long line of B's.

cmr=v1.7.4:reviewer=jsquyres:subject=Do a better job of reporting bindings
Handle the case of nodes that do not report cores, and thus our default binding policy will fail even though binding is supported by defaulting to not binding on those nodes.

Thanks to Paul Hargrove for reporting the problem on NetBSD.

cmr=v1.7.4:reviewer=jsquyres:subject=Handle the case of nodes that do not report cores
Clean up many references to "rank": usually change to "process" and/or
specifically delineate that we're referring to the process' rank in
MPI_COMM_WORLD.

Refs #4068
Ensure slurm properly dies on abnormal termination

cmr=v1.7.4:reviewer=jsquyres:subject=Ensure slurm properly dies on abnormal termination
Use the portable "$?" macro to build examples.

Thanks to Paul Hargrove for reporting the issue.

cmr=v1.7.4:reviewer=rhc:subject=Fix use of non-portable Makefile macro