More GCC compatibility/warning fixes (#3118)

* Wrap >4-char literals in a MULTI_CHAR macro

Modern compilers do not support CW's non-standard behavior with
>4 char literals. We can, however, use a constexpr function to
compute the u64 values directly. This leaves <=4 char literals
unchanged.

* Replace non-pointer usages of NULL with 0

* Define NULL to nullptr on C++11 and above

* Fix more -Wpointer-arith and -Woverflow warnings

* Replace u32/s32 with uintptr_t/intptr_t where appropriate

* JSUOutputStream: Overload all standard int types

src/d/d_event_manager.cpp

@@ -1245,7 +1245,7 @@ void* dEvent_manager_c::getMySubstanceP(int staffId, const char* dataname, int d
 int dEvent_manager_c::getMySubstanceNum(int staffId, const char* dataname) {
     dEvDtData_c* data = getMyDataP(staffId, dataname, FALSE);
     if (data == NULL) {
-        return NULL;
+        return 0;
     } else {
         return data->getNumber();
     }