rec: Add support for dumping the in-memory RPZ zones to a file

[rgacogne]

Short description

Allow dumping the current content of a RPZ zone via rec_control dump-rpz <zone name> <filename>. The content is dumped as a DNS zone and can be directly loaded afterwards with rpzFile().

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)