Bug 18321: Correctly calculate switch_onsite_checkout

Bug 18321: Correctly calculate switch_onsite_checkout

Silly error here, the $switch_onsite_checkout is not correctly
calculated, it should not only be set to the value of the syspref, but
meet all the conditions.

Test plan:
Set a circ rule with number of on-site checkout = 2 and number of
checkouts = 2.
Switch on both ConsiderOnSiteCheckoutsAsNormalCheckouts and SwitchOnSiteCheckouts

Do a regular checkout
Do another regular checkout
At this point you have reach the maximum number of checkouts allowed.
Not try another one.
=> Without this patch you will not get the warning
=> With this patch you will see it

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>