Add tlsenum package and add version/cipher enumeration into the checker

tlsenum package probes a remote endpoint with one ClientHello
per (version, cipher) pair via utls, so the checker can report the
exact set the server accepts rather than only the suite Go's stdlib
happens to negotiate. Probe accepts an Upgrader callback so STARTTLS
dialects plug in without tlsenum learning about them; the checker
bridges its existing dialect registry through upgraderFor.

checker/starttls_xmpp.go

@@ -22,13 +22,20 @@ func starttlsXMPPServer(conn net.Conn, sni string) error {
 	return starttlsXMPP(conn, sni, "jabber:server")
 }
 
+// xmppPreTLSReadLimit caps the bytes the XML decoder may pull from an
+// untrusted peer before the TLS handshake. The legitimate pre-TLS exchange
+// (<stream:stream> opening + <stream:features> + <proceed/>) is well under
+// 1 KiB; 64 KiB is generous for non-malicious servers while bounding memory
+// against a peer that streams unbounded XML to exhaust the prober.
+const xmppPreTLSReadLimit = 64 * 1024
+
 func starttlsXMPP(conn net.Conn, sni, ns string) error {
 	header := fmt.Sprintf(`<?xml version='1.0'?><stream:stream xmlns='%s' xmlns:stream='http://etherx.jabber.org/streams' version='1.0' to='%s'>`, ns, sni)
 	if _, err := io.WriteString(conn, header); err != nil {
 		return fmt.Errorf("write stream header: %w", err)
 	}
 
-	dec := xml.NewDecoder(conn)
+	dec := xml.NewDecoder(&io.LimitedReader{R: conn, N: xmppPreTLSReadLimit})
 
 	// Read the inbound <stream:stream> opening and its <stream:features>.
 	// A peer that opens with <stream:error/> (or anything other than features)