We need to do a bit more validation when we get a server name, but don't want
to be re-doing it all over the shop, so factor out a separate
parse_and_validate_server_name, and do the extra validation.

Also, use it to verify the server name in the config file.

Make sure that server_names used in auth headers are sane, and reject them with
a sensible error code, before they disappear off into the depths of the system.