Select next file among files in current tags table.
A first argument of t (prefix arg, if interactive) initializes to the
beginning of the list of files in the tags table. If the argument is
neither nil nor t, it is evalled to initialize the list of files.
Non-nil second argument NOVISIT means use a temporary buffer
to save time and avoid uninteresting warnings.
Value is nil if the file was already visited;
if the file was newly read in, the value is the filename. (fn &optional INITIALIZE NOVISIT)