IFLIB updates

• unconditionally enable BUS_DMA on non-x86 architectures
• speed up rxd zeroing via customized function
• support out of order updates to rxd's
• add prefetching to hardware descriptor rings
• only prefetch on 10G or faster hardware
• add seperate tx queue intr function
• preliminary rework of NETMAP interfaces, WIP

Submitted by: Matt Macy <mmacy@nextbsd.org>
Sponsored by: Limelight Networks