Refactor unmapped buffer address handling.

• Use pointer assignment rather than a combination of pointers and flags to switch buffers between unmapped and mapped. This eliminates multiple flags and generally simplifies the logic.
• Eliminate b_saveaddr since it is only used with pager bufs which have their b_data re-initialized on each allocation.
• Gather up some convenience routines in the buffer cache for manipulating buf space and buf malloc space.
• Add an inline, buf_mapped(), to standardize checks around unmapped buffers.

In collaboration with: mlaier
Reviewed by: kib
Tested by: pho (many small revisions ago)
Sponsored by: EMC / Isilon Storage Division