Clean up parsing.

[Stebalien]

We should always construct requests with the NewRequest function and should handle everything like CheckArguments there. Currently, we end up calling CheckArguments multiple times and have to defend against this. Bleh.

[Stebalien]

It's actually worse. I've fixed the double check here but we also check in cli's parseArgs function. However, we do so with completely different logic (and without blocking on stdin).

Really, we should do this in stages:

1. Parse but don't check. Unfortunately, we still need access to the tree of commands to distinguish between arguments and subcommands.
2. Handle help (before checking anything else).
3. Check options and arguments.

This should also allow us to deduplicate some of this functionality.