template::multirow foreach feature, which loops over the rows in a multirow, setting local vars, executing a code block, and then resetting the values in the foreach. Useful in conjunction with template::multirow extend.
Added -extend switch to db_multirow and removed Arjun's regsub hack in same. db_multirow also sets the varname:columns list, and it checks that columns stay the same when you append. I've also added documentation, as per SDM #1486