Blocks not already in {blocks} table can't be enabled with install_set_block()

If you use your installation profile to create something that has a block attached, like an aggregator feed, I think you should be able to enable that block using install_set_block(). However, at present, there is only an "UPDATE" sql statement, which of course cannot enable blocks that don't already exist in the {blocks} table. The workaround is just to write a db_query to insert the block manually before running install_set_block, but I think we should just have the function itself do that if it's necessary.