Bug 4405 - Overdues block check out

Bug 4405 - Overdues block check out

[Documentation note by RM:

This patch adds a new system preference, OverduesBlockCirc,
that can take one of three values:

  - noblock:      checkouts permitted even if patrons
                  have overdue items (default)
  - confirmation: circ operator asked to confirm checkout
  - block:        checkouts to patrons who have overdue
                  items are forbidden

]

Squashed commit of the following:

commit 6a1f66e0686a14d8a89abfc3fe5978dabd0b7af7
Author: Chris Cormack <chrisc@catalyst.net.nz>
Date:   Mon Apr 26 10:27:39 2010 +1200

    Tidy up ready to send patch

commit 4d1398df18dcce4fd888cf17a0e2955fdf6ee1e4
Author: Chris Cormack <chrisc@catalyst.net.nz>
Date:   Mon Apr 26 10:26:15 2010 +1200

    Bug 4405 - tidy up

commit 3daeb71bc6b690e18dda96aa3c767c2bb0521038
Author: Chris Cormack <chrisc@catalyst.net.nz>
Date:   Mon Apr 26 10:02:04 2010 +1200

    Bug 4405 - Overdues block checkout

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>