r26451: Janitorial: fix warnings in lib/registry/

r26451: Janitorial: fix warnings in lib/registry/

This does not fix the discarded qualifier warnings in tests, as the test data
is currently passed as const. Jelmer wants to provide a test function that
passes non-const test data, thus allowing for a cleaner way to fix those
warnings.
(This used to be commit 46dfa63d4f7381c5c6ce3f4b8b0bd9aa9e16950c)