httpserver: Prevent TLS client authentication bypass in 3 ways (#2099)

- Introduce StrictHostMatching mode for sites that require clientauth
- Error if QUIC is enabled whilst TLS clientauth is configured
  (Our QUIC implementation does not yet support TLS clientauth, but
  maybe it will in the future - fixes #2095)
- Error if one but not all TLS configs for the same hostname have a
  different ClientAuth CA pool

caddyhttp/httpserver/siteconfig.go

@@ -36,6 +36,16 @@ type SiteConfig struct {
 	// TLS configuration
 	TLS *caddytls.Config
 
+	// If true, the Host header in the HTTP request must
+	// match the SNI value in the TLS handshake (if any).
+	// This should be enabled whenever a site relies on
+	// TLS client authentication, for example; or any time
+	// you want to enforce that THIS site's TLS config
+	// is used and not the TLS config of any other site
+	// on the same listener. TODO: Check how relevant this
+	// is with TLS 1.3.
+	StrictHostMatching bool
+
 	// Uncompiled middleware stack
 	middleware []Middleware