Make ZVOL writes in device mode support IO_SYNC flag.

MFC after:	1 month
Improve cache control support, including DPO/FUA flags and the mode page.

At this moment it works only for files and ZVOLs in device mode since BIOs
have no respective respective cache control flags (DPO/FUA).

MFC after:	1 month
Sponsored by:	iXsystems, Inc.
Rename new to newval in inline asm code, to avoid clashes with C++ new.
Also rename cmp to cmpval just to keep the asm variable names similar to
the C variable names.
Add support for Mode Page Policy (0x87) VPD page.
Revert r269474. Special keyboard combinations should be handled by separate
sysctls.
Oops, missed piece of r271311.
- Add missing "
- Sort sections
Minor mdoc nit.
Report that DPO and FUA bits are supported after r271311.