contrib/cleanup: loosen type-checking in auto_free

- advantage:    `auto_free anyType *foo;` works (for anyType != char)
- disadvantage: `auto_free anyType bar;` is also accepted,
  though I expect such problems will be relatively easy to debug.

contrib/cleanup.h

@@ -11,8 +11,8 @@
 #include <stdlib.h>
 
 #define auto_free __attribute__((cleanup(_cleanup_free)))
-static inline void _cleanup_free(char **p) {
-	free(*p);
+static inline void _cleanup_free(const void *p) {
+	free(*(char **)p);
 }
 #define auto_close __attribute__((cleanup(_cleanup_close)))
 static inline void _cleanup_close(int *p) {