During debugging of print() it can be useful to dig in the pipeline of commands called underground. For this reason there is the -debug option which is not documented. Even if the user discover that, it is impossible to access the generated temporary files to further debug outside octave

I am proposing this simple patch which:

1) add some info to the help about the hidden -debug option

2) add a new -keep-temp-files option to preserve temporary files, in this way it is possible to investigate on the intermediate results of the underground pipeline

In principle the -keep-temp-files is superfluous and everything can be driven only by -debug itself. 

Here is attached the proposed patch