Strip control characters from email header fields. They shouldn't be in there.

SPAMMERS insert these either intentionally or mistakenly and they can defeat
security checks (like mailauth.js).

src/sbbs3/mailsrvr.c

@@ -3432,6 +3432,7 @@ static void smtp_thread(void* arg)
 				continue;
 			}
 			/* RFC822 Header parsing */
+			strip_ctrl(buf, buf);	/* There should be no control characters in header fields */
 			if(startup->options&MAIL_OPT_DEBUG_RX_HEADER)
 				lprintf(LOG_DEBUG,"%04d SMTP %s",socket, buf);