Restore handling of physical processors in rankfiles

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-pick of:
open-mpi/ompi@2a90788
open-mpi/ompi@d0704ef

@jsquyres I know you are thrilled, but I don't know anyone else that can handle it.