daemon: allow per-listener servername= and serverport=

daemon: allow per-listener servername= and serverport=

Being able to specify per-listener servername= with the
`--listen' CLI arg is helpful for running an Tor .onion NNTP
server with the same config file (and thus
publicinbox.nntpserver) as a public-facing NNTP endpoint.

For HTTP, servername= and serverport= allow overriding the
SERVER_NAME and SERVER_PORT PSGI environment variables,
respectively.  These are useful for generating full URLs
for clients which don't send the HTTP `Host:' header.

These currently have no effect aside from wasting memory for
POP3 and IMAP listeners.  serverport= isn't used by NNTP,
either.