Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Use an sx lock to serialize writes since they update the checksum.
- Remove D_NEEDGIANT as the rtc drivers already have their own locks, so
this doesn't need Giant.

MFC after: 1 week

Convert amd64 and i386 to share the atrtc device driver.

Split /dev/nvram driver out of isa/clock.c for i386 and amd64. I have not
refactored it to be a generic device.
Instead of being part of the standard kernel, there is now a 'nvram' device
for i386/amd64. It is in DEFAULTS like io and mem, and can be turned off
with 'nodevice nvram'. This matches the previous behavior when it was
first committed.