httpserver: max_certs now forces On-Demand TLS even if name is known

Original feature request in forum:
https://forum.caddyserver.com/t/caddy-with-specific-hosts-but-on-demand-tls/1704?u=matt

Before, Caddy obtained certificates for every name it could at startup.
And it would only obtain certificates during the handshake for sites
defined with a hostname that didn't qualify at startup (like
"*.example.com" or ":443"). This made sense for most situations, and
helped ensure that certificates were obtained as early and reliably as
possible.

With this change, Caddy will NOT obtain certificates for hostnames it
knows at startup (even if they qualify) if OnDemand is enabled.

But I think this change generalizes well, because a user who specifies
max_certs is deliberately turning on On-Demand TLS, fully aware of
the consequences. It seems dubious to ignore that config when the user
deliberately put it there. We'll see how this goes.

caddyhttp/httpserver/https.go

@@ -23,6 +23,9 @@ func activateHTTPS(cctx caddy.Context) error {
 
 	// place certificates and keys on disk
 	for _, c := range ctx.siteConfigs {
+		if c.TLS.OnDemand {
+			continue // obtain these certificates on-demand instead
+		}
 		err := c.TLS.ObtainCert(c.TLS.Hostname, operatorPresent)
 		if err != nil {
 			return err
@@ -65,15 +68,15 @@ func markQualifiedForAutoHTTPS(configs []*SiteConfig) {
 }
 
 // enableAutoHTTPS configures each config to use TLS according to default settings.
-// It will only change configs that are marked as managed, and assumes that
-// certificates and keys are already on disk. If loadCertificates is true,
-// the certificates will be loaded from disk into the cache for this process
-// to use. If false, TLS will still be enabled and configured with default
-// settings, but no certificates will be parsed loaded into the cache, and
-// the returned error value will always be nil.
+// It will only change configs that are marked as managed but not on-demand, and
+// assumes that certificates and keys are already on disk. If loadCertificates is
+// true, the certificates will be loaded from disk into the cache for this process
+// to use. If false, TLS will still be enabled and configured with default settings,
+// but no certificates will be parsed loaded into the cache, and the returned error
+// value will always be nil.
 func enableAutoHTTPS(configs []*SiteConfig, loadCertificates bool) error {
 	for _, cfg := range configs {
-		if cfg == nil || cfg.TLS == nil || !cfg.TLS.Managed {
+		if cfg == nil || cfg.TLS == nil || !cfg.TLS.Managed || cfg.TLS.OnDemand {
 			continue
 		}
 		cfg.TLS.Enabled = true