Fix incorrectly ordered arguments to xcalloc

Fix incorrectly ordered arguments to xcalloc

The gnulib xcalloc call has the following signature:
 void *calloc(size_t N, size_t S);
where N is the number of objects and S is the size of each object.
In many places, we had these arguments transposed. In many implementations
this doesn't matter since the two arguments are simply multiplied together.
However, on some systems this can cause problems (ie crash), if S is zero.
This change fixes all calls where the size was being passes as the first
argument instead of the second.