auth: somewhat cleanup global symbols

[zeha]

Short description

• Make many things in auth-main.cc static
• Rename some of the single/two-letter globals to longer names, but mostly those local to auth-main.cc
• Move SOAData, DNSBackend to auth-only include files

I've deliberately ignored the symbols used across many files (QC, PC, ...), in the hope this could still go into 4.7.0.

Checklist

I have:

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

[zeha]

Rebased to fix conflict.