Use proper C indexing.

No more of this pseudo-Fortran indexing. It's odd and causes unnecessary
work (to be fixed).
Remove unnecessary allocations and loops.
Add missing header.

ctype.h is required for isspace.
Use MPI_STATUS_IGNORE in MPI calls.

The status is not used, so this saves us an extra variable or two.
Fix some simple type-conversion warnings.
build: Make changes to scotch build permanent.

Every time you configured with internal scotch, you'd get modifications
to its Makefiles which git would show as modified. This is troublesome
whenever you want to commit or change branches, because you'd have to
remember not to keep those changes.

The changes to the scotch Makefiles are independent of any options you
might pass to configure (except that they aren't done if you aren't
using internal scotch). We can just make them permanently in git,
especially because the changes have been made upstream in scotch as
well.