Fixes #3685: Cleanup some pmi2 things

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r28853 [[BR]]
Cleanup and rename of platform files

r28854 [[BR]]
Refs #3685. Check error code returned by PMI2_Info_GetJobAttr.


r28855 [[BR]]
Remove set-but-unused variable

Refs #3685


r28856 [[BR]]
fix condition in common/pmi init that c caused pmi to fail if PMI2_Init succeeds

r28857 [[BR]]
One last Cray-inspired fix...

Refs #3685