Fixes `hide_edge_borders smart` when gaps are in use.
Implements `hide_edge_borders smart_no_gaps` and `smart_borders
on|no_gaps|off`.
Since `smart_borders on` is equivalent to `hide_edge_borders smart`
and `smart_borders no_gaps` is equivalent to `hide_edge_borders
smart_no_gaps`, I opted to just save the last value set for
`hide_edge_borders` and restore that on `smart_borders off`. This
simplifies the conditions for setting the border.
