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.
This allows git-pages and Caddy to efficiently use the same connection
for many pipelined requests, which I hope will reduce contention when
some bot decides to send fifty requests in the same millisecond.
This commit also changes built-in Caddy configuration to use HTTP/2
cleartext only when talking to the backend.
