[DEV-634] registrar: add script binding for REGISTER reply function

This permits explicitly sending an appropriate reply after a save(),
even from non-request route.
[DEV-634] registrar: add reg_ prefix to save and lookup

Kamailio's total lack of module namespacing means prefixes should
probably be added to most of the worst offenders in the easily
collidable names department.
[DEV-634] registrar: remove REGISTER 200 OK response from reg_save()