Time and check commands setting fat16/32 labels and UUIDs (#754684)

Replace open coding of the creation of the operation details for the
mlabel command used to set the label and UUID with calls to
FileSystem::execute_command() which will do it all.  This also results
in the commands getting a time and check mark displayed in the operation
details.

Bug 754684 - Updates to FileSystem:: and Utils::execute_command()
             functions