Option to add add reject-unconstrained-dependencies to the freeze file

#5475 landed `--reject-unconstrained-dependencies`, which is particularly useful with freeze files. However, presently, either the option needs to be specified at every invocation, or you need to know to put it in one of the configuration files, which might affect more than you want it to. I think that it makes more sense for new-freeze to grow an `--add-reject-unconstrained-dependencies` option (or some suitably bikeshedded name), which will add that to the freeze file.