Restore the sysctl changes from 223676 and 227309 lost in the previous

Restore the sysctl changes from 223676 and 227309 lost in the previous
import:
- Add read-only sysctls for all of the tunables supported by the igb and
  em drivers.
- Make the per-instance 'enable_aim' sysctl truly per-instance by having it
  change a per-instance variable (which is used to control AIM) rather
  than having all of the per-instance sysctls operate on a single global
  variable.

While here, restore the previously existing hw.igb.rx_processing_limit
tunable as it is very useful to be able to set a default tunable that
applies to all adapters in the system.