Provide explicit interfaces according to the Fortran routine interface specifications.

[jeffhammond]

From MPI 4.0 19.1.2:

Provide explicit interfaces according to the Fortran routine interface specifications. This module therefore guarantees compile-time argument checking for all arguments which are not TYPE(*), with the following exception:
Only one Fortran interface is defined for functions that are deprecated as of MPI-3.0. This interface must be provided as an explicit interface according to the rules defined for the mpi module, see Section 19.1.3.