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_kantera_icon_meter.cpp

@@ -31,9 +31,9 @@ void dKantera_icon_c::initiate() {
     dPaneClass_showNullPane(scrn);
     mpKanteraIcon->setScreen(scrn);
 
-    mpParent = new CPaneMgr(scrn, 'kan_m_n', 2, NULL);
+    mpParent = new CPaneMgr(scrn, MULTI_CHAR('kan_m_n'), 2, NULL);
 
-    mpGauge = new CPaneMgr(scrn, 'yellow_m', 0, NULL);
+    mpGauge = new CPaneMgr(scrn, MULTI_CHAR('yellow_m'), 0, NULL);
 }
 
 void dKantera_icon_c::setAlphaRate(f32 alphaRate) {