Overhaul bus_dma(9) usage and fix various things.

 o Separate TX/RX buffer DMA tag from TX/RX descriptor ring DMA tag.
 o Separate RX buffer DMA tag from common buffer DMA tag. RX DMA
   tag has different restriction compared to TX DMA tag.
 o Add 40bit DMA address support.
 o Adjust TX/RX descriptor ring alignment to 64 bytes from 256
   bytes as documented in datasheet.
 o Added check to ensure TX/RX ring reside within a 4GB boundary.
   Since TX/RX ring shares the same high address register they
   should have the same high address.
 o TX/RX side bus_dmamap_load_mbuf_sg(9) support.
 o Add lock assertion to vge_setmulti().
 o Add RX spare DMA map to recover from DMA map load failure.
 o Add optimized RX buffer handler, vge_discard_rxbuf which is
   activated when vge(4) sees bad frames.
 o Don't blindly update VGE_RXDESC_RESIDUECNT register. Datasheet
   says the register should be updated only when number of
   available RX descriptors are multiple of 4.
 o Use __NO_STRICT_ALIGNMENT instead of defining VGE_FIXUP_RX which
   is only set for i386 architecture. Previously vge(4) also
   performed expensive copy operation to align IP header on amd64.
   This change should give RX performance boost on amd64
   architecture.
 o Don't reinitialize controller if driver is already running. This
   should reduce number of link state flipping.
 o Since vge(4) drops a driver lock before passing received frame
   to upper layer, make sure vge(4) is still running after
   re-acquiring driver lock.
 o Add second argument count to vge_rxeof(). The argument will
   limit number of packets could be processed in RX handler.
 o Rearrange vge_rxeof() not to allocate RX buffer if received
   frame was bad packet.
 o Removed if_printf that prints DMA map failure. This type of
   message shouldn't be used in fast path of driver.
 o Reduce number of allowed TX buffer fragments to 6 from 7. A TX
   descriptor allows 7 fragments of a frame. However the CMZ field
   of descriptor has just 3bits and the controller wants to see
   fragment + 1 in the field. So if we have 7 fragments the field
   value would be 0 which seems to cause unexpected results under
   certain conditions. This change should fix occasional TX hang
   observed on vge(4).
 o Simplify vge_stat_locked() and add number of available TX
   descriptor check.
 o vge(4) controllers lack padding short frames. Make sure to fill
   zero for the padded bytes. This closes unintended information
   disclosure.
 o Don't set VGE_TDCTL_JUMBO flag. Datasheet is not clear whether
   this bit should be set by driver or write-back status bit after
   transmission. At least vendor's driver does not set this bit so
   remove it. Without this bit vge(4) still can send jumbo frames.
 o Don't start driver when vge(4) know there are not enough RX
   buffers.
 o Remove volatile keyword in RX descriptor structure. This should
   be handled by bus_dma(9).
 o Collapse two 16bits member of TX/RX descriptor into single 32bits
   member.
 o Reduce number of RX descriptors to 252 from 256. The
   VGE_RXDESCNUM is 16bits register but only lower 8bits are valid.
   So the maximum number of RX descriptors would be 255. However
   the number of should be multiple of 4 as controller wants to
   update 4 RX descriptors at a time. This limits the maximum
   number of RX descriptor to be 252.

Tested by:	Dewayne Geraghty (dewayne.geraghty <> heuristicsystems dot com dot au)
		Carey Jones (m.carey.jones <> gmail dot com)
		Yoshiaki Kasahara (kasahara <> nc dor kyushu-u dot ac dotjp)

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