srm: Make srmRmdir S2 compliant

S2 expects that attempts to remove a directory to which files are
uploaded fail. The test enforcing this behaviour is
usecase.RmdirBeingPutInto.

This patch implements the required behaviour.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6154/