ix: Import Intel ixgbe-2.5.15

MSI and legacy interrupt (line based interrupt) are working as of this
commit.  polling(4) will be supported and MSI-X will be brought back in
the later commits.

ixgbe is renamed to ix, which is consistent w/ the interface name, and
this makes 'ifconfig ix0' work as expected.

Local changes:
- Reorganize RX and TX ring data structure; nuke useless 'queue'.
- Rework RX and TX ring creation, destroy, initialization and cleanup.
- Reorganize interrupt related data structure.  This also prepares for
  bringing MSI-X back.
- Rework interrupt allocation.
- Rework MSI and legacy interrupt handler.
- Enable RSS (2 RX rings) even if MSI or legacy interrupt is used.
- Apply DragonFly specific RSS configuration.
- Partially rework RX code path.
- Enable TX header write-back.
- Rework TX code path and TX watchdog mechanism.
- Rework busdma(9) related bits.
- Rework ifnet.if_ioctl method.
- Reorganize initialize and stop code.
- Reorganize attach and detach code.
- Code clean up.