xattr-util: rework getxattr_at_malloc()

Let's return the size in a return parameter instead of the return value.
And if NULL is specified this tells us the caller doesn't care about the
size and expects a NUL terminated string. In that case look for an
embedded NUL byte, and refuse in that case.

This should lock things down a bit, as we'll systematically refuse
embedded NUL strings now when we expect strings.

src/basic/cgroup-util.h

@@ -264,7 +264,7 @@ int cg_get_owner(const char *path, uid_t *ret_uid);
 
 int cg_set_xattr(const char *path, const char *name, const void *value, size_t size, int flags);
 int cg_get_xattr(const char *path, const char *name, void *value, size_t size);
-int cg_get_xattr_malloc(const char *path, const char *name, char **ret);
+int cg_get_xattr_malloc(const char *path, const char *name, char **ret, size_t *ret_size);
 /* Returns negative on error, and 0 or 1 on success for the bool value */
 int cg_get_xattr_bool(const char *path, const char *name);
 int cg_remove_xattr(const char *path, const char *name);