res_monitor.c: Don't emit a warning about 'X' being unrecognized.

Code was added in 030f7d41 to warn if an unrecognized option was
passed to an application, but code in Monitor was taking advantage of
the fact that the application would silently accept an invalid option.

We now recognize the invalid option but we don't do anything if it's
set.

Fixes #639

(cherry picked from commit 6938b7f962)

res/res_monitor.c

@@ -672,6 +672,19 @@ enum {
 	MON_FLAG_DROP_IN =  (1 << 2),
 	MON_FLAG_DROP_OUT = (1 << 3),
 	MON_FLAG_BEEP =     (1 << 4),
+
+	/*!
+	 * \brief Dummy option to silence WARNINGs.
+	 *
+	 * Application argument processing was updated to emit a WARNING when an
+	 * unrecognized option was encountered. Previously, unrecognized options
+	 * were silently ignored which was taken advantage of by code in this module
+	 * to "stamp out" problematic application arguments and replace them with 'X.'
+	 *
+	 * So to silence the warning we add 'X' as a valid option and just don't do
+	 * anything if it's set.
+	 */
+	MON_FLAG_DUMMY =    (1 << 5),
 };
 
 enum {
@@ -685,6 +698,7 @@ AST_APP_OPTIONS(monitor_opts, {
 	AST_APP_OPTION('i', MON_FLAG_DROP_IN),
 	AST_APP_OPTION('o', MON_FLAG_DROP_OUT),
 	AST_APP_OPTION_ARG('B', MON_FLAG_BEEP, OPT_ARG_BEEP_INTERVAL),
+	AST_APP_OPTION('X', MON_FLAG_DUMMY),
 });
 
 /*!