Post-testnet system contract follow-ups

[TheJudii]

Moves the contract changes that were added after the testnet deployment out of PR #64's mainnet-upgrade path.

This draft PR preserves them for separate review:

• cap votebpout penalty at 168 hours and add the corresponding overflow guard/tests
• require proxy auth for regproxy and add the corresponding regression test

Base is the cleaned PR #64 branch so this PR should show only the post-testnet follow-up changes.