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.