fix all implicit conversion between signed/unsigned pointers

sadly the C language does not specify any such implicit conversion, so
this is not a matter of just fixing warnings (as gcc treats it) but
actual errors. i would like to revisit a number of these changes and
possibly revise the types used to reduce the number of casts required.

src/stdio/vfprintf.c

@@ -149,7 +149,7 @@ static void pop_arg(union arg *arg, int type, va_list *ap)
 
 static void out(FILE *f, const char *s, size_t l)
 {
-	__fwritex(s, l, f);
+	__fwritex((void *)s, l, f);
 }
 
 static void pad(FILE *f, char c, int w, int l, int fl)