Bug 15443 - Re-code RESERVESLIP as HOLD_SLIP

Bug 15443 - Re-code RESERVESLIP as HOLD_SLIP

It's a bit confusing to have RESERVESLIP named "Hold Slip", we should
finish the job and recode the slip.

Test Plan:
1) Apply this patch
2) Run updatedatabase.pl
3) Try printing a hold slip, you should note no difference
   from pre-patch behavior

Followed test plan. The slip is triggered if you want to check out
an item that is reserved / on hold for another patron. Works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>