ftp/ls: columnate output attached to a tty, else don't

ftp/ls: columnate output attached to a tty, else don't

'bup ftp ls' and 'bup ls' currently behave in a different manner.

'bup ftp ls' always formats its output in columns regardless of whether
the program's stdout is a tty or not.

'bup ls' always prints one name on each line.

Make both of those commands behave the same. By using lib/bup/helpers'
istty1 variable, decide to format in columns when outputting to a tty,
and to output one file name per line when the output is not a tty.

Signed-off-by: Gabriel Filion <lelutin@gmail.com>