Fix a problem with combining MPI::LargeCount and PETSc.

Fix a problem with combining MPI::LargeCount and PETSc.

PETSc, in petsclog.h, implements logging by redefining all MPI functions as
macros, e.g.,

    #define MPI_Send_c(buf,count,datatype,dest,tag,comm) \
    ((petsc_send_ct++,0) || PetscMPITypeSize((count),(datatype),(&petsc_send_len)) || MPI_Send_c((buf),(count),(datatype),(dest),(tag),(comm)))

This only works when all MPI declarations are made before these macro
definitions. Hence this library won't work with PETSc unless we include headers
in the right order.

Work around this by removing the MPI_ prefix from these function definitions.
Since we put these functions in our own namespace there is no need for them to
have the same names as the normal MPI functions.