igb: Define macros for wreg_nsegs, instead of using magic number
igb: Move IGB_FLAG_TSO_IPLEN0 into TX rings

Avoid extra access to softc and leave enough space for TX ring specific
flags.
igb: Add per-TX ring enable flag.

For TX rings' whose enable flag are not set, which could happen during
polling entering/exiting, its corresponding subqueue will be purged.
igb: Add IGB_TSS_DEBUG kernel option
igb: Add per-TX ring tx_pkt sysctl node, if IGB_TSS_DEBUG is enabled
if: Add function to set subqueue count
igb: Setup subqueue count properly
if: Add power of 2 mask based CPUID to subqueue mapping
igb: Utilize power of 2 mask based CPUID to subqueue mapping
if: Fix wrong assertion condition in ifq_set_subq_mask