Refactor the 'max-cache-size' configuration handling

Refactor the 'max-cache-size' configuration handling

Extract the inline max-cache-size logic from configure_view() into
reusable helpers: configure_max_cache_size(), default_max_cache_size(),
max_cache_size_as_percent(), and sanitized_max_cache_size().

Move DNS_CACHE_MINSIZE and DNS_ADB_MINADBSIZE to public headers and
remove the SIZE_AS_PERCENT sentinel.