Replace -proxy-mode=<bool> with -proxy=<enum>

The proxy flag really needs three values, not just true/false. Establish the flag as an enum now rather than after v0.5-rc is cut to prevent unnecessary churn. Once the `activeSize` cluster config is actually supported, we can implement the "fallback" proxy mode.
Hmm, making `proxyFlagValues` an array complicates the call to `strings.Join`. Not doing it.