do sentinel detection in Fortran

[jeffhammond]

detect sentinels in Fortran layer rather than C so we don't need to capture sentinel addresses and therefore don't need to initialize MPI in Fortran to setup this state.

[jeffhammond]

also, i'm not even detecting MPI_BOTTOM anywhere right now 🤣

[jeffhammond]

it is impossible to determine if a buffer is MPI_BOTTOM in Fortran, without resorting to LOC(), which is non-standard.

[jeffhammond]

design tests:

• https://github.com/jeffhammond/HPCInfo/blob/master/mpi/abi/sentinel.F90
• https://github.com/jeffhammond/HPCInfo/blob/master/mpi/abi/sentinel2.F90
• https://github.com/jeffhammond/HPCInfo/blob/master/mpi/abi/sentinel3.F90