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>

src/userdel.c

@@ -763,7 +763,7 @@ static void user_cancel (const char *user)
 	}
 	argv[0] = cmd;
 	argv[1] = user;
-	argv[2] = (char *)0;
+	argv[2] = NULL;
 	(void) run_command (cmd, argv, NULL, &status);
 }