Use output()/error() instead of print

We shouldn't be using print in the core API, with
the possible (?) exception of error/abort messages.

mininet/cli.py

@@ -373,7 +373,7 @@ class CLI( Cmd ):
     def do_links( self, _line ):
         "Report on links"
         for link in self.mn.links:
-            print( link, link.status() )
+            output( link, link.status() )
 
     def do_switch( self, line ):
         "Starts or stops a switch"
@@ -407,7 +407,7 @@ class CLI( Cmd ):
 
         if first in self.mn:
             if not args:
-                print( "*** Enter a command for node: %s <cmd>" % first )
+                error( "*** Enter a command for node: %s <cmd>" % first )
                 return
             node = self.mn[ first ]
             rest = args.split( ' ' )