srm: Clean up mkdir and rmdir implementations

Improves error codes and error messages of srmMkdir and srmRmdir
operations. Improves performance of srmRmdir.

By moving some functionality into the Storage class, the logic
could be simplified and made faster.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6120/