parsers: rename YY_NULL as YY_NULLPTR to avoid conflicts with Flex

Flex also defines YY_NULL (to 0).  Avoid gratuitous conflicts.

* data/c.m4 (b4_null_define): Rename YY_NULL as YY_NULLPTR.

* data/glr.c, data/lalr1.cc, data/location.cc, data/variant.hh,
* data/yacc.c, src/parse-gram.c, tests/actions.at, tests/c++.at,
* tests/cxx-type.at, tests/glr-regression.at, tests/headers.at,
* tests/push.at, tests/regression.at:
Adjust.

tests/headers.at

@@ -229,7 +229,7 @@ AT_CHECK([[$PERL -n -0777 -e '
       |YY_IGNORE_MAYBE_UNINITIALIZED_(?:BEGIN|END)
       |YY_INITIAL_VALUE
       |YY_\w+_INCLUDED
-      |YY_NULL
+      |YY_NULLPTR
       |(defined|if)\ YYDEBUG
       )\b}{}gx;
   while (/^(.*YY.*)$/gm)