Free results of varobj_get_type and type_to_string

Free results of varobj_get_type and type_to_string

varobj_get_type and type_to_string return an allocated string, which is
not freed at a couple of places.

New in v2:
 * Rename char * type to type_name.
 * Free in all cases in update_type_if_necessary.

gdb/ChangeLog:

	* mi/mi-cmd-var.c (mi_cmd_var_info_type): Free result of
	varobj_get_type.
	(varobj_update_one): Same.
	* varobj.c (update_type_if_necessary): Free curr_type_str and
	new_type_str.
	(varobj_get_type): Specify in comment that the result needs to be
	freed by the caller.