tor-guardmgr: Temporarily reintroduce VanguardConfig.
As discussed on #tor-dev, I'm reintroducing VanguardConfig for now.
The VanguardConfig specifies what mode (full/lite/disabled) the
VanguardMgr should run in. We currently don't have a separate modes
for HS clients and HS services.
We shouldn't actually need a VanguardConfig at all, so this is just
a (hopefully!) short- or medium-term fix until we sort out #1382 (which
might involve making breaking changes to our reconfigure() APIs).
Closes #1272 (closed)