minor type-safety and variable init changes in init.cpp

- explicit init of pcoinsdbview and pwalletMain (even if not needed, as
  globals are init to NULL, it seems cleaner)
- remove check if (pwalletMain) in Shutdown() as delete is valid even if
  pwalletMain is NULL
- use correct type (%d / %u) in log and help messages
Didn't know that, but doesn't it make sense to still try to match what the variable type is? It makes lazy and perhaps at some time we drop tinyformat.