dnsproxy: first bits of refactoring data types, global variables, simpler functions

- move all type declarations to the top of the unit to have them all in
  one place, same for global variables
- introduce enums for having more descriptive identifiers for some of
  the DNS header constants
- remove unnecessary zero initializations for global variables
- move variable declarations into more local scopes where possible (e.g.
  in for loops). Shorter lifetimes of variables can make the code more
  easy to follow.
- avoid some repetitive code sequences like `cache_free_ipv4()` by
  moving them into separate functions
- use const variables in parameters where possible to make certain
  guarantees of function calls more clear and avoid erroneous
  assignments.

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