Clarify array indexes vs. row numbers

Clarify array indexes vs. row numbers

There was some confusion between "index" as used for the model->files[] array,
and node->index as used for our 1-based row numbers.  Now we use "index" only
for indices in the model->files[] array, and node->row for row numbers.  Functions
and variables are renamed to clarify whether they refer to indexes or rows.

Signed-off-by: Federico Mena Quintero <federico@novell.com>