Exposing sysctls for ixgbe

[hiren panchasara]

$ sysctl hw.igb
hw.igb.rxd: 4096
hw.igb.txd: 4096
hw.igb.enable_aim: 1
hw.igb.enable_msix: 1
hw.igb.max_interrupt_rate: 8000
hw.igb.buf_ring_size: 4096
hw.igb.header_split: 0
hw.igb.num_queues: 1
hw.igb.rx_process_limit: 100
$ sysctl hw.ix
sysctl: unknown oid 'hw.ix': No such file or directory

I thought it would be nice to have these things exposed. So I copied them
from igb:
http://people.freebsd.org/~hiren/ixgbe_sysctls.txt

Changes for if_igb.c is to expose correct auto-tuned value for a running
system for "hw.igb.num_queues", which is not the case right now.

Thanks to markj@ for help/pointers.

Please let me know if the diffs look okay.

cheers,
Hiren