Add support for bounce buffers to _bus_dmamap_load_buffer, which is the

backend for bus_dmamap_load_mbuf and bus_dmamap_load_uio.

- Increaes MAX_BPAGES to 512.  Less than this causes fxp to quickly runs out
  of bounce pages.
- Add an argument to reserve_bounce_pages indicating wether this operation
  should fail or be queued for later processing if we run out of memory.
  The EINPROGRESS return value is not handled properly by consumers of
  bus_dmamap_load_mbuf.
- If bounce buffers are required allocate minimum 1 bounce page at map
  creation time.  If maxsize was small previously this could get truncated
  to 0 and the drivers would quickly run out of bounce pages.
- Fix a bug handling the return value of alloc_bounce_pages at map creation
  time.  It returns the number of pages allocated, not 0 on success.
- Use bus_addr_t for physical addresses to avoid truncation.
- Assert that the map is non-null and not the no bounce map in
  add_bounce_pages.

Sponsored by:	DARPA, Network Associates Laboratories

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