this makes handling the relative "root" directory not a special case (".").
mountpoints now always begin with a slash. before, "/foo" and "foo" were
in two separate namespaces, which was weird and confusing; the two now
mean the same thing. there are no more special "default mountpoints"; just
mount to "/". "@path" mounts are now "/@path", and they do show up in
GetDirListing("/") (but, as before, opening "/@foo/bar" will never create
a "@foo/bar" path in a non-"@foo" mountpoint)
Glenn Maynard
209040e02d