Harden STARTTLS handlers and add per-dialect tests

Bound line reads with readLineLimited to prevent a peer from exhausting
memory by withholding line terminators, wrap previously bare error
returns for consistent context, surface XML decoder Skip errors, and
replace the goto in the XMPP feature scan with a labeled break. New
starttls_test.go exercises SMTP/IMAP/POP3/XMPP/LDAP success and
not-advertised paths through net.Pipe-mocked servers.

checker/starttls_smtp.go

@@ -60,7 +60,7 @@ func readSMTPGreeting(r *bufio.Reader) error {
 func readSMTPResponse(r *bufio.Reader) ([]string, error) {
 	var out []string
 	for {
-		line, err := r.ReadString('\n')
+		line, err := readLineLimited(r)
 		if err != nil {
 			return out, err
 		}