admin: Add array output

Several cell commands return binary objects. These are used internally, but are
also exposed through the admin interface. They are converted to string form using
Object#toString, however for arrays this only produces an object reference.

This patch adds output of the array content to the admin door.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6851/