Correct error message for unsupported ifupdown2 attributes

Corrected a minor error in how we handle unsupported config attributes.
Instead of

     error: not enough arguments for format string

we will now print this:

     warning: peerlink.4000: unsupported attribute 'clagd-foobar'
(cherry picked from commit 44c6004a33d59ee234f8ee3d5f450e158c9e5bdc)
(cherry picked from commit f2c2321301d05e88d3c9ab9c26e6d4e256c884e9)

ifupdown2/ifupdown/ifupdownmain.py

@@ -534,7 +534,8 @@ class ifupdownMain(ifupdownBase):
                     break
             if not found:
                 err = True
-                self.logger.warn('%s: unsupported attribute \'%s\'' %attrname) 
+                self.logger.warn('%s: unsupported attribute \'%s\'' \
+                                 % (ifaceobj.name, attrname))
                 continue
         return err