BUF/BIO work, for removing the requirement of KVA mappings for I/O

BUF/BIO work, for removing the requirement of KVA mappings for I/O
requests.

Stage 1 of 8:

	o Replace the b_pages member of the BUF structure with an embedded
	  XIO (b_xio).  The XIO will be used for managing the BUF's page
	  lists.

	o Initialize the XIO at two main (only) points: 1) the pbuf code,
	  which is used by the NFS code to create a temporary buffer; and
	  bufinit(9), which is used by the rest of the BUF/BIO consumers.

Discussed-with: 	Matthew Dillon <dillon@apollo.backplane.com>,