Fix extension callbacks invocations.

Make sure that if we call an extension callback it is done via a
wrapper to be able to catch problems.
Always set up entry pointer for extension.
Make sure extension.flags is up to date.

When we retrieve rules from iptables, reapply all parameters from the
entry, so that the (match|target).(m|t)flags get updated. This commit
fixes #22.
Whitespace fixes.