Support --tlshostname option

Useful for confirming behaviour when using a different hostname to any
found in the certificate -- ideally, fix the certificates.  But we're a
tool used by admins debugging, so let people debug!
Interactive 'debug' command if --debug

It can be useful to toggle debug off after connection has been
established and authenticated, etc.  So at the REPL, provide a `debug`
command, but only if invoked with `--debug`.