Add free_default() to free a struct defaults pointer so we have a

single place where we free the defaults.  A pointer to the previous
Default's binding may be passed in to avoid freeing an already free
binding.

plugins/sudoers/parse.h

@@ -268,6 +268,7 @@ void free_member(struct member *m);
 void free_members(struct member_list *members);
 void free_privilege(struct privilege *priv);
 void free_userspec(struct userspec *us);
+struct member_list *free_default(struct defaults *def, struct member_list *binding);
 
 /* match_addr.c */
 bool addr_matches(char *n);