cccooommmbbbiiinnneeesss pppnnnmmm___rrreeeaaadddpppnnnmmmiiinnniiittt((())), pppnnnmmm___aaallllllooocccaaarrrrrraaayyy((())) and pppnnnmmm___rrreeeaaadddpppnnnmmmrrrooowww((())).
Unlike the equivalent functions in PBM, PGM, and PPM, it returns the
format so you can tell what type the file is.
void pnm_writepnminit( FILE* fp, int cols, int rows, xelval maxval, int