callagent: Respond with a failure to OPTIONS requests

There is no proper implementation yet, also no way for an application
to tweak that behavior but there is also no need for that right now.

callagent/SIPCallAgent.st

@@ -37,6 +37,17 @@ SIPByeRequest extend [
     ]
 ]
 
+SIPOptionsRequest extend [
+    sipDispatchNewDialog: aDialog on: aUserAgent [
+        <category: 'OsmoSIP-Callagent'>
+        aUserAgent
+            respondWith: 481
+            phrase: 'Call Leg/Transaction Does Not Exist'
+            on: self
+            dialog: aDialog.
+    ]
+]
+
 Object subclass: SIPTransport [
     | queue handler |
     <category: 'OsmoSIP-Callagent'>