Refs #4512: Move 31372, r31373 to v1.8 branch (mpi_f08 fixes)

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r31455 [[BR]]
mpi_f08 fixes from Craig's review:

* Superfluous use of MPI_User_function in comm_create_keyval_f08.F90
* Missed adding "value" keyword to function pointer arguments in pmpi
  C interfaces

Submitted by Craig, reviewed by Jeff.

Refs #4512