scanner: add the Interface.plainname property

Returns "connection" for the "ei_connection" interface, i.e. the
interface stripped of any component prefix.

proto/ei-scanner

@@ -317,6 +317,16 @@ class Interface:
         """
         return Interface.mangle_name(self.protocol_name, self.mode)
 
+    @property
+    def plainname(self) -> str:
+        """
+        Returns the plain name of the interface, i.e. this returns
+        "pointer", "handshake", etc. without the "ei_" or "eis_" prefix.
+        """
+        if self.protocol_name.startswith("ei_"):
+            return f"{self.protocol_name[3:]}"
+        return self.protocol_name
+
     @staticmethod
     def mangle_name(name: str, component: str) -> str:
         """