Make sure something is written to stdout.

When calling the save() callback of an extension, make sure at least
a new line is written to stdout - if the extension has had no parameters
passed to it, then it might have caused python-iptables to hang, waiting
for input on the pipe.
Fix exception format string parameters.
Fix handling of flags.

Now we always pass 0 as flags for extension callbacks, and just store
the result back into mflags/tflags. This way in the end we should get
the correct flags map, that can be passed to e.g. final_check().