Update flag parsing

[RebeccaMahany]

We use ff, which uses flag for command-line arg parsing, right now. This hampers our ability to parse flags on launcher startup because flag cannot handle unknown command-line flags -- we have to wait until main has bootstrapped the most recent launcher that will know about all current flags.

There are limited other options that allow for handling unknown flags. pflag and go-flags are the two I found, but neither seems to be maintained at the moment, and neither handles config files or environment variables the way that ff does.

We will likely want to move to a new solution that addresses all our needs, while remaining backward-compatible for older installations of launcher.

I began tentative use of pflag in #1512 for autoupdate library lookup functionality -- I chose pflag over go-flags because it has more widespread use and has been updated relatively more recently. If we update to a new option, we should update that usage as well.