make `MPIR_ContextMaskToStr` non-static

This permits external users/developers to use this routine for debugging
context ID allocation problems in the field.  At some point we may want
to expose this info through the `MPI_T_` interface.

Note that this isn't the way I would do it if I were building this stuff
from scratch, I'm just taking a direct path from what we've got to
something more useful.

Reviewed-by: dinan
add `MPIR_ContextMaskStats` debug routine

This should be useful for debugging context ID allocation issues in the
wild.

Reviewed-by: dinan
report context ID counts on allocation failure

When we run out of context IDs, we now report how many context IDs are
free on each process in the input communicator.  This should make it
easier to identify the causes of bad allocation patterns in the wild.

Reviewed-by: dinan