Unify the RPZ log messages

[pettai]

Describe the bug
Very remotely related is a nit to #628

First hit on the QNAME log says "rpz: applied ..."
https://github.com/NLnetLabs/unbound/blob/master/services/rpz.c#L1405
While the response IP hit log says "RPZ applied ..."
https://github.com/NLnetLabs/unbound/blob/master/respip/respip.c#L1304

To reproduce
Compare QNAME and Response IP RPZ hit logs (example here)
#628

Expected behavior
Log uniformly "rpz: ..."

System:

• Unbound version: 1.15.0
• OS: Ubuntu
• unbound -V output:

[gthess]

"rpz:" was chosen for solving this.