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.
It actually looks not terrible for -proxy:

```
% ./bin/etcd -proxy=snarf
invalid value "snarf" for flag -proxy: invalid value
```