portFromAddr resolves numeric and named ports (":http", ":https")
without touching /etc/services or the resolver. listenersOverlap
follows kernel rules: same port plus a wildcard host on either side,
or same port plus identical specific host, both count as collision;
different specific hosts on the same port do not. Set an explicit
viper default for tls_letsencrypt_listen so a minimal config still
resolves to ":http".
Updates #3227
Kristoffer Dalby
6031194c36