[Mpi3-tools] Updates to DLL Discovery API proposal

[Jeff Squyres]

Per the call today, I updated https://svn.mpi-forum.org/trac/mpi-forum-web/wiki/MPI3Tools/dllapi 
  to remove all references to MPIR_debug_gate, and changed the text to  
reflect that the debugger may read the value at any time.  If the  
symbol does not exist, then the MPI doesn't support the DLL and the  
debugger shouldn't bother checking again.

If the symbol exists and has a NULL value, the debugger should try  
again later.  If the symbol exists and does not have a NULL value,  
then the debugger can try the DLLs listed in the array.

I will likely not update the MPI handle debugging API proposal until  
after we get some feedback from the Forum.

-- 
Jeff Squyres
Cisco Systems