- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

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

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

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

Remove clause 3 from Izumi Tsutsui's licenses.

Obtained from: NetBSD

Style changes

Obtained from: NetBSD (mc146818reg.h)

Export mc146818_def_{read,write}() so the front-end can make use
of them if needed.

After some input from bde@ and rereading the datasheet use a MTX_SPIN
mutex instead of a MTX_DEF one in order to defer preemption while
reading the date and time registers. If we don't manage to read them
within the time slot where we are guaranteed that no updates occur we
might actually read them during an update in which case the output is
undefined.

o mc146818(4):
- Add locking.
- Account for if the MC146818_NO_CENT_ADJUST flag is set we don't need
to check wheter year < POSIX_BASE_YEAR.
- Add some comments about mapping the day of week from the range the
generic clock code uses to the range the chip uses and which I meant
to add in the initial version.
- Minor clean-up, use __func__ instead of hardcoded function names in
error strings.

o in the rtc(4) front-end additionally:
- Don't leak resources in case mc146818_attach() fails.
- Account for ebus(4) defaulting to SYS_RES_MEMORY for the memory
resources since ebus.c rev. 1.22.

Start each of the license/copyright comments with /*-, minor shuffle of lines

Add a driver back end for MC146818 and compatible clocks based on the
respective NetBSD driver for use with the genclock interface.
It's first use will be on sparc64 but it was also tested on alpha with
a preliminary patch to switch alpha to use the genclock code together
with this driver instead of the respective code in alpha/alpha/clock.c
and the rather MD mcclock(4). Using it on i386 and amd64 won't be that
hard but some changes/extensions to improve the genclock code in general
should be done first, e.g. add locking and make it easier to access the
NVRAM usually coupled with RTCs.