init.cpp: log fee estimates filename on error

- small changes to Shutdown(), buffer ``__func__``, which is now used in
  all LogPrintf() calls and format for better readability
- order using namespace alpabetically
Please don't assign `__func__` to a variable here. `__func__` *is* already a hidden variable added to every function by the compiler but just using it as-is makes it easier to pick out.