bup: pass TTY width through to sub-command and remote

bup: pass TTY width through to sub-command and remote

As we rewrite the sub-command's stderr, it cannot determine
the width. As a result, e.g. "bup ls" will always columnate
only to 70 columns, instead of using the entire width. Fix
this by passing the real width through in an environment
variable, just like we already pass through the fact that
it's a TTY.

Also pass it through on the ssh command line so that if you
run 'bup on ... ls' it actually behaves right.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>