cmd/ftp: Hide .dotfiles by default (-a shows them)

cmd/ftp: Hide .dotfiles by default (-a shows them)

Normally in FTP sites, files beginning with a dot are hidden from a list
(ls) command by default. Also, using the argument '-a' makes the list
show hidden files.

The current 'bup ftp' implementation does not behave so. Make it hide
hidden files by default, as expected, and show hidden files when '-a' or
'--all' is specified to the 'ls' command.

All unknown switches will make bup ftp show the ls command usage.

Users can also give 'ls --help' to obtain the usage string.

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