misc: Implement pdns::UniqueFilePtr

misc: Implement pdns::UniqueFilePtr

Using a deleter instead of `decltype(&fclose)` has two big advantages:
- the deleter is included in the type and does not have to be passed
  when creating a new object (easier to use, less memory usage, in theory
  better inlining)
- we avoid the annoying `ignoring attributes on template argument ‘int (*)(FILE*)’`
  warning from the compiler, which is there because `fclose` is tagged as `__nonnull((1))`