ls: -1 for non-tty

[mknos]

• The manual explains that if output is not a tty, ls will infer the -1 flag (i.e. print one entry per line)
• The old filemode check code did not work on my Linux system but this patch does
• I tested this with a pipe into cat, and with a redirect into a file

[briandfoy]

changes: infer -1 switch automatically if the output filehandle is not a tty