Improved URL display on MOTD

[Tronic]

Sanic would use any host setting for URL construction. This PR fixes the common cases of normal localhost and wildcard addresses, for which we instead display localhost. Similarly for UNIX sockets, as that format works e.g. with curl. URL to wildcard doesn't work at all, and connecting localhost by IP rather than domain name can be problematic with TLS if not otherwise.

Additionally, hides the port number if standard ports are used.

[Tronic]

Setting host to localhost instead of IP probably needs to be done in server startup rather than MOTD only, to make url_for work as expected without extra configuration. This is tricky, leaving in draft mode for now.

[ahopkins]

This is tricky, leaving in draft mode for now.

Maybe also time to handle the whole server name issue. 😉

[ahopkins]

This whole bit feels to me like logic we should probably abstract to a utility function somewhere. Don't we do this elsewhere also?

[Tronic]

Maybe also time to handle the whole server name issue. 😉

That's a can of worms, for sure. Would be useful to first identify all the problems that we are solving here, and things affected by them.

• External URL forming (btw, should use app.url_for in MOTD too).
  • Scheme and host and port may be different than locally seen
  • Currently by SERVER_NAME, what are the concerns about this (URL vs. host only, host:port)
  • If apps can be externally mounted also at some path other than /, internal URLs are also affected
  • In global context where the app is not yet running env is available, app.run args are not!
• Host (request header) fallback
  • Local or remote values here? Start giving 400 Bad Request to those without valid header?
• Listening address(es) to bind (IPs, unix path)
  • Might be displayed separately on MOTD, should always be displayed inside the box?
• CLI and app.run interfaces for specifying these things?
• What is --host used for?
  • Binding or hostname, both?
  • Changing the default to localhost would make sense and avoid some special cases
• Are we adding a --listen shorthand for doing all at once and/or listening to more than one
  • Could specify multiple to bind, but the server can only have one host:port with app.url_for
  • Listen to IPv4 and IPv6 wildcards if :8000 (nothing before colon) is specified (matches Caddy)
  • Default to 80/443 if plain hostname/ip is given?
  • Auto-TLS if given name and no port, background server on 80? (redirect/ACME, matches Caddy)

What'd I miss?