Do all primitive work globally

As the Lua interface works globally, we really should as well
from the TeX side. This also keeps things sensible in terms of the
save stack.

luatexconfig.tex

@@ -25,12 +25,12 @@
   \directlua{tex.enableprimitives("",
     {"pageheight", "pagewidth", "outputmode"})}%
   \ifx\pdfoutput\undefined
-    \let\pdfoutput\outputmode
-    \let\pdfpageheight\pageheight
-    \let\pdfpagewidth\pagewidth
+    \global\let\pdfoutput\outputmode
+    \global\let\pdfpageheight\pageheight
+    \global\let\pdfpagewidth\pagewidth
     \def\do#1{%
       \ifx\relax#1\else
-        \expandafter\def\csname pdf#1\endcsname{\pdfvariable #1}%
+        \expandafter\xdef\csname pdf#1\endcsname{\pdfvariable #1}%
         \expandafter\do
       \fi
     }%
@@ -45,7 +45,7 @@
   \global\let\dvimode\undefined
   \def\do#1{%
     \ifx\relax#1\else
-      \global\expandafter\let\csname pdf#1\endcsname\relax
+      \global\expandafter\let\csname pdf#1\endcsname\undefined
       \expandafter\do
     \fi
   }%