Support physical processor ids in rankfile

(cherry picked from commit 2a9078872439ac3d818df2a78132484553a1fc50)
Restore handling of physical processors in rankfiles. Note that the prior implementation was likely incorrect as it falsely assumed that physical core indices were unique, which isn't always true. Stipulate that physical rankfiles can only include PU numbers, and bind the result to the core that contains that physical PU. Update the mpirun man page to cover the new use-case.

(cherry picked from commit d0704ef118599e97d398511734abc3e5cc40b898)
Minor changes per Jeff's request on PR for 1.8.4
Merge pull request #103 from rhc54/cmr/new-rankfile

Restore handling of physical processors in rankfiles