Fix to avoid a denial-of-service attack if the display width is

Fix to avoid a denial-of-service attack if the display width is
enormous.  Also, clean up the code a bit by removing duplicate code.

(init_column_info): Remove forward decl; no longer needed.
(calculate_columns): New function, that contains code that used
to be common to print_many_per_line and print_horizontal.
(print_many_per_line, print_horizontal): Use it.
(decode_switches): Set max_idx here, not in calculate_columns.
(print_current_files): Don't call init_column_info; calculate_columns
now does that.
(init_column_info): Don't allocate a lot more space than is needed
to represent the current set of files.  Allocate all the new
size_t cells in one call to xnmalloc, rather than a row at a time.