telnet: make rfc2217 handling more correct

This implements several things that were missing before:

 - Send initial DO and WILL for COM_PORT option.
 - Handle a IAC IAC sequence as single in-band '\xff'.
 - Reply WILL/DO for non-COM_PORT options with DONT/WONT.
 - Stop interpreting escape sequences at their end.
 - Add necessary bound checks.

This closes #5. It's still not optimal though. Missing bits include for
example:

 - tracking of WILL/WONT/DO/DONT
 - Only start with COM_PORT subnegotiation once the other side acked.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Author: Uwe Kleine-König

microcom.h

@@ -120,8 +120,17 @@ int do_script(char *script);
 
 #define dbg_printf(fmt,args...)  ({ if (debug) printf(fmt ,##args); })
 
+/*
+ * Some telnet options according to
+ * https://www.iana.org/assignments/telnet-options/telnet-options.xhtmls
+ */
+
+#define TELNET_OPTION_BINARY_TRANSMISSION		0
+#define TELNET_OPTION_ECHO				1
+#define TELNET_OPTION_SUPPRESS_GO_AHEAD			3
+#define TELNET_OPTION_COM_PORT_CONTROL			44
+
 /* RFC2217 */
-#define COM_PORT_OPTION		 44
 #define SET_BAUDRATE_CS		  1
 #define SET_DATASIZE_CS		  2
 #define SET_PARITY_CS		  3