Multiop has the potential to be really useful as an I/O pattern simulator, in that it can perform almost any operation.  In theory, we could process iotrace logs from a user application, and then generate command strings for multiop.  This works great, but we very quickly run in to the limit of the maximum command input size (the shell limit).

So, add the ability for multiop to take command input from a file.  This is implemented a little awkwardly in order to make it entirely optional and not change the many existing tests using multiop.