Cleanup the sensor code:

* use the global flags for linux and apple being found instead of re-doing the case statements

* update select procedure to ignore components that measure the same thing (e.g., resusage and sigar), taking the higher priority module

cmr=v1.7.5:reviewer=jsquyres:subject=Cleanup the sensor code
Update platform file

cmr=v1.7.4:reviewer=ompi-gk1.7