[Mpi-22] Ticket #46: Add const Keywordto the C bindings-implementation

[Rajeev Thakur]

One reason to deal with this issue in 2.2 versus 3 is that if we decide one
way or the other now itself (whether const should or should not be in the
MPI interface), it helps all other interfaces being defined in MPI 3 to know
now itself whether they need to include const in the right places. If we
delay until 3, the new interfaces will have to go back and add const at the
last minute, or keep 2 sets of interfaces concurrently active until the end
-- with const and without const. 

Regarding implementation, if you start with the mpi.h provided in the ticket
and let the compiler catch you, it can be done (although it is still
tedious). How far down you propagate const depends on how much time you are
willing to spend. 

Rajeev