Call NULL by its name

In variadic functions we still do the cast.  In POSIX, it's not
necessary, since NULL is required to be of type 'void *', and 'void *'
is guaranteed to have the same alignment and representation as 'char *'.
However, since ISO C still doesn't mandate that, and moreover they're
doing dubious stuff by adding nullptr, let's be on the cautious side.
Also, C++ requires that NULL is _not_ 'void *', but either plain 0 or
some magic stuff.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

libmisc/env.c

@@ -41,7 +41,7 @@ static const char *const forbid[] = {
 	"PATH=",
 	"SHELL=",
 	"SHLIB_PATH=",
-	(char *) 0
+	NULL
 };
 
 /* these are allowed, but with no slashes inside
@@ -50,7 +50,7 @@ static const char *const noslash[] = {
 	"LANG=",
 	"LANGUAGE=",
 	"LC_",			/* anything with the LC_ prefix */
-	(char *) 0
+	NULL
 };
 
 /*