ixgbe sysctl hardware defaults

[smh]

Summary

Added hw.ix.flow_control which enables the default flow_control of all ix interfaces to be set in loader.conf

Added hw.ix.advertise_speed which enables the default advertised_speed of all ix interfaces to be set in loader.conf

Made enable_aim device independent based on hw.ix.enable_aim default.

Test Plan

Confirm flow control, advertise speed and aim can be configured by the global hw.ix sysctls.

[erj]

I haven't compile or functional tested, but the contents of the diff look good. It's a good idea to split out the actual setting of the advertised speed and flow control settings from the sysctl stuff.