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

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

Prepare for the removal of set_rcvar() by changing the rcvar=
assignments to the literal values it would have returned.

The concept of set_rcvar() was nice in theory, but the forks
it creates are a drag on the startup process, which is especially
noticeable on slower systems, such as embedded ones.

During the discussion on freebsd-rc@ a preference was expressed for
using ${name}_enable instead of the literal values. However the
code portability concept doesn't really apply since there are so
many other places where the literal name has to be searched for
and replaced. Also, using the literal value is also a tiny bit
faster than dereferencing the variables, and every little bit helps.

Add gptboot_enable rc variable, which allows to turn gptboot reporting off in
case user wants to implement his own actions and doesn't want the attributes to
vanish.

Obtained from: Wheel Systems Sp. z o.o. http://www.wheelsystems.com
MFC after: 3 days

Add gptboot script that is responsible for:
- looking for partition with 'bootonce' attribute alone (without 'bootme'
attribute), removing it and logging that we successfully booted from this
partition.
- looking for partitions with 'bootfailed' attribute, removing it and
logging that we failed to boot from this partition.

Reviewed by: arch (Message-ID: <20100917234542.GE1902@garage.freebsd.pl>)
Obtained from: Wheel Systems Sp. z o.o. http://www.wheelsystems.com
MFC after: 2 weeks