freebsd-rustdate clean

This provides some ability to clean up things freebsd-rustdate would do. Or at least, it’s a placeholder for such things. Perhaps more will grow here someday.

At the moment, the only supported action is --pending, which cleans up update state. That is, it cleans up and forgets info about a pending fetch or upgrade. It’s hard to imagine a time when you’d need to explicitly do that. But just in case…