Added Wireshark extcap command line parameter interface to mstpcap utility.

Tested with Wireshark on Windows (mostly working).
To use extcap, run Wireshark and go to the About-dialog.  Find a tab located there named "Folders". Locate the extcap search path. Copy the mstpcap.exe to that folder, which may not exist.
Restart Wireshark, and look for "BACnet MS/TP on COMx" interfaces.
Configure the interface to change baud rate.
Capture directly from the interface.

bacnet-stack/ports/linux/rs485.c

@@ -624,6 +624,14 @@ void RS485_Initialize(
     printf("=success!\n");
 }
 
+void RS485_Print_Ports(void)
+{
+    /* note: format for Wireshark ExtCap */
+    //printf("interface {value=/dev/ttyUSB%u}"
+    //    "{display=MS/TP Capture on /dev/ttyUSB%u}\n", i, i);
+    /* FIXME: add code to print ports */
+}
+
 #ifdef TEST_RS485
 #include <string.h>
 int main(

bacnet-stack/ports/linux/rs485.h

@@ -67,6 +67,8 @@ extern "C" {
 
     void RS485_Cleanup(
         void);
+    void RS485_Print_Ports(
+        void);
 
 #ifdef __cplusplus
 }

bacnet-stack/ports/win32/rs485.c

@@ -489,6 +489,26 @@ void RS485_Check_UART_Data(
     }
 }
 
+/*************************************************************************
+* Description: print available COM ports
+* Returns: none
+* Notes: none
+**************************************************************************/
+void RS485_Print_Ports(
+    void)
+{
+    unsigned i = 0;
+
+    /* try to open all 255 COM ports */
+    for (i = 1; i < 256; i++) {
+        if (RS485_Interface_Valid(i)) {
+            /* note: format for Wireshark ExtCap */
+            printf("interface {value=COM%u}"
+                "{display=BACnet MS/TP on COM%u}\n", i, i);
+        }
+    }
+}
+
 #ifdef TEST_RS485
 
 #include "mstpdef.h"

bacnet-stack/ports/win32/rs485.h

@@ -70,6 +70,8 @@ extern "C" {
 
     bool RS485_Interface_Valid(
         unsigned port_number);
+    void RS485_Print_Ports(
+        void);
 
 
 #ifdef __cplusplus