Make the ixgbe tunables now match their sysctl counterparts.

Previously the tunables and sysctls had different names for example:
hw.ixgbe.enable_aim => hw.ix.enable_aim

Anyone using ixgbe tunables should ensure they update /boot/loader.conf.

This is a direct commit to stable as the changes to sysctls in head
already fix this issue in a different way.

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