Add the buffer number as an argument to sy#stop()

sy#stop() is called by the BufDelete autocmd so b: variables may not
correspond to the buffer actually being deleted.  Instead, we must use
<abuf> to determine which buffer is being deleted and pass that into
sy#stop().

There are some ripple effects, causing sy#sign#remove_all_signs to also
gain a buffer number argument.

Signed-off-by: James McCoy <vega.james@gmail.com>