* cli: Change command structure, add help subcommand (#328)
* cli: improve subcommand structure
- make help command as normal subcommand
- add flag usage message for each command
* cmd: Refactor subcommands and command line help; make commands pluggable
* Begin WIP integration of HTTP/3 support
* http3: Set actual Handler, make fakeClosePacketConn type for UDP sockets
Also use latest quic-go for ALPN fix
* Manually keep track of and close HTTP/3 listeners
* Update quic-go after working through some http3 bugs
* Fix go mod
* Make http3 optional for now
Along with several other changes, such as renaming caddyhttp.ServerRoute
to caddyhttp.Route, exporting some types that were not exported before,
and tweaking the caddytls TLS values to be more consistent.
Notably, we also now disable automatic cert management for names which
already have a cert (manually) loaded into the cache. These names no
longer need to be specified in the "skip_certificates" field of the
automatic HTTPS config, because they will be skipped automatically.
