Change function pointers to function prototypes

[Gnimuc]

fix #694

[giordano]

Unfortunately this doesn't fix my troubles in #684

(mvapich) pkg> add https://github.com/Gnimuc/MPI.jl.git#fix-694
     Cloning git-repo `https://github.com/Gnimuc/MPI.jl.git`
    Updating git-repo `https://github.com/Gnimuc/MPI.jl.git`
    Updating registry at `~/.julia/registries/General.toml`
   Resolving package versions...
    Updating `~/tmp/mvapich/Project.toml`
  [da04e1cc] ~ MPI v0.20.5 `~/.julia/dev/MPI` ⇒ v0.20.5 `https://github.com/Gnimuc/MPI.jl.git#fix-694`
    Updating `~/tmp/mvapich/Manifest.toml`
  [da04e1cc] ~ MPI v0.20.5 `~/.julia/dev/MPI` ⇒ v0.20.5 `https://github.com/Gnimuc/MPI.jl.git#fix-694`
Precompiling environment...
  1 dependency successfully precompiled in 11 seconds. 12 already precompiled.

julia> using MPI

julia> MPI.Init(;threadlevel=:single)
MPI.ThreadLevel(0)

julia> MPI.Datatype(Char)
MPI.Datatype(Char): 

julia> MPI.Finalize()

[1197027] signal (11.1): Segmentation fault
in expression starting at REPL[5]:1
MPIR_Call_attr_delete at /lustre/software/mvapich2/gcc11/2.3.6/lib/libmpi.so (unknown line)
MPIR_Attr_delete_list at /lustre/software/mvapich2/gcc11/2.3.6/lib/libmpi.so (unknown line)
Allocations: 3851567 (Pool: 3848167; Big: 3400); GC: 7
Segmentation fault (core dumped)

[t-bltg]

I'm trying to come up with a representative test.

MPI_Comm_create_errhandler might be a good candidate.

Porting the example from http://mpi.deino.net/mpi_functions/MPI_Comm_create_errhandler.html to test/test_errorhandler.jl shouldn't be too hard, trying that now.

[Gnimuc]

The default backend works locally on my machine:

@giordano which backend should I use to reproduce this segfault?

[giordano]

All problems I reported in #684 are on Ookami, most of them with MVAPICH2. I have no clue how to reproduce them outside of that environment.

[simonbyrne]

Thanks for fixing this.