Avoid pointless (but harmless) actions on unmanaged pages.

Reviewed by:	kib
Sponsored by:	EMC / Isilon Storage Division
Streamline format extensions. Either the compiler supports them, and
we enable them and format wordings. Or it doesn't, and we disable
format warnings because the kernel uses the extensions pervasively.
Delete pp_isadma. It isn't use, and the code that used it has been
commented out (temporarily) since 1998 when this driver hit the
tree. Also, no need to compute the ethernet header and then never use
it.
Disable all inline warnings on gcc >= 4.3. Not sure exactly where the
cutover is, but we need better tools to cope with inline tuning per
compiler version than we have. This is a quick bandaid until such
tools are around.
ins is only set and unused, but only when we're not doing software
single stepping. Only set it when we're doing that by bending
style(9) rules a little to avoid even worse #ifdef soup.
Only install the boot loader if it actually exists. This is a stop-gap
change, since larger changes to use geom more exclusively to create
partitions is in th works.
create_$ARCH_diskimage never really took off. Collapse back down to
just one.