util: Establish global logger object.

The object encapsulates logging configuration, and in a later commit,
set up routines will also be moved into the class.

src/util.h

@@ -66,7 +66,7 @@ bool SetupNetworking();
 template<typename... Args>
 bool error(const char* fmt, const Args&... args)
 {
-    LogPrintStr("ERROR: " + tfm::format(fmt, args...) + "\n");
+    LogPrintf("ERROR: %s\n", tfm::format(fmt, args...));
     return false;
 }