[meshcop] move border agent into core (#2771)

This commit moves the border agent service into OpenThread core.

Border agent, commissioner, and joiner shares a single secure CoAP agent,
because they will not be active at the same time.

Other changes include:
- Replaced TMF proxy with UDP proxy, which is more generalized.
- Changed OT_ERROR_NONE string to "OK".
- Defined a special interface id -1 to differentiate packets received by
  host side.

examples/drivers/windows/otApi/otApi.cpp

@@ -3440,7 +3440,7 @@ otThreadErrorToString(
     switch (aError)
     {
     case OT_ERROR_NONE:
-        retval = "None";
+        retval = "OK";
         break;
 
     case OT_ERROR_FAILED: