copy: extend check for mount point crossing

copy: extend check for mount point crossing

We do this checks as protection against bind mount cycles on the same
file system. However, the check wasn't really effective for that, as
it would only detect cycles A → B → A this way. By using
fs_is_mount_point() we'll also detect cycles A → A.

Also, while we are at it, make these file system boundary checks
optional. This is not used anywhere, but might be eventually...

Most importantly though add a longer blurb explanation the why.