Fix for #286 for pre-existing rules at the start of a chain

In #286 we fixed rule offset detection for existing managed and
unmanaged rules, but in the case where the first rule in a chain was
unmanaged, managed rules were still being inserted under it.

This patch changes it so that if the first rule detected for offset is
unmanaged, then we should insert before that for more consistent
behavior.
Merge pull request #303 from hunner/fix_unmanaged

Fix for #286 for pre-existing rules at the start of a chain