Hide or show ../ depending on g:netrw_retmap

When I care about mouse enough to set g:netrw_retmap, show ../ entries
as they are the only means of going up with the mouse.
Preserve # when browsing with <LeftMouse> as well

Locally re-implemented s:NetrwLeftMouse(), so :keepalt protection could
be added to it.
Merge branch 'mouse'