More specifically, remove the dedicated HTTP datapath for health
checks and verify the entire stack, from TLS frontend to S3 backend.
Verifying too little has resulted in a small outage recently when
the pages listener got misconfigured but the health listener happily
accepted connections like normal. This would not happen now that
the health check uses port 443, too.
This is done using reflection to avoid boilerplate and potential desync
of the two configuration interfaces. The `[[wildcards]]` section did
not fit well into the "splat every config key" paradigm, so it is
unmarshalled as a whole from a JSON payload in an environment variable.
This commit also splits up the `Config` type into small per-section
struct types and removes most references to the global `config` in
favor of passing pointers to sections around.
A new option, `-print-config-env-vars`, shows the names and types of
all of the available configuration knobs.
