dns.cc: use pdns::views::UnsignedCharView

dns.cc: use pdns::views::UnsignedCharView

Includes minor cleanup and additions to make UnsignedCharView usable for this use case.
Supersedes #14356
Fixes
/usr/include/c++/v1/__fwd/string_view.h:22:41: warning: 'char_traits<unsigned char>' is deprecated: char_traits<T> for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided for a temporary period. It will be removed in LLVM 19, so please migrate off of it. [-Wdeprecated-declarations]