SERVER-10726 Orthogonalize debugging and optimization options

Introduce new flags --dbg and --opt which permit independent control over optimization and
debugging. The old --d, --dd, and --release flags were not independent, and had surprising
interactions, particularly on Windows.

The old flags are still supported in a legacy mode for --d and --dd builds, cannot be mixed
with the new.

For builds specifying none of --dbg, --opt, --d, or --dd, there should be no behavior change,
except on Windows. On that platform, a no-flags-specified build will now default to
optimization enabled, whereas before it did not.