Bug 18936: (follow-up) Make possible to have unlimited value for holds_per_day

Bug 18936: (follow-up) Make possible to have unlimited value for holds_per_day

Without this it is not possible to make for a specific patron category
or itemtype a more specific unlimited holds_per_day rule if you have a
catch all rule (categorycode = *, itemtype = *) with limited
holds_per_day value.

Signed-off-by: Minna Kivinen <minna.kivinen@hamk.fi>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>