Big MFC of the physbio changes necessary for unmapped I/O.  These changes
have been in production at Netflix for several months with significant
success.

MFC r246713:

Reform the busdma API so that new types may be added without modifying
every architecture's busdma_machdep.c.  It is done by unifying the
bus_dmamap_load_buffer() routines so that they may be called from MI
code.  The MD busdma is then given a chance to do any final processing
in the complete() callback.

MFC r249538:
Some compilers issue a warning when wider integer is casted to narrow
pointer.  Supposedly shut down the warning by casting through
uintptr_t.

MFC r251479:
Simplify the checking of flags for cam_periph_mapmem().  This gets rid of
a lot of code redundancy and grossness at very minor expense.

MFC r251837:
MFC r251842:
Add infrastructure for doing compatibility shims, as has been sorely
needed for the last 10 years.  Far too much of the internal API is
exposed, and every small adjustment causes applications to stop working.
To kick this off, bump the API version to 0x17 as should have been done
with r246713, but add shims to compensate.  Thanks to the shims, there
should be no visible change in application behavior.

Submitted by:	kib, jeffr
Approved by:	kib
Obtained from:	Netflix