tag-utils: use the tag_opt_list_t as talloc context, if possible.

The memory usage discipline of tag_op_list_t is never to free the
internal array of tag operations before freeing the whole list, so it
makes sense to take advantage of hierarchical de-allocation by talloc.

By not relying on the context passed into tag_parse_line, we can allow
tag_op_list_t structures to live longer than that context.