Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

The NetBSD Foundation has granted permission to remove clause 3 and 4 from
the software.

Obtained from: NetBSD

Now that all platforms use genclock, shuffle things around slightly
for better structure.

Much of this is related to <sys/clock.h>, which should really have
been called <sys/calendar.h>, but unless and until we need the name,
the repocopy can wait.

In general the kernel does not know about minutes, hours, days,
timezones, daylight savings time, leap-years and such. All that
is theoretically a matter for userland only.

Parts of kernel code does however care: badly designed filesystems
store timestamps in local time and RTC chips almost universally
track time in a YY-MM-DD HH:MM:SS format, and sometimes in local
timezone instead of UTC. For this we have <sys/clock.h>

<sys/time.h> on the other hand, deals with time_t, timeval, timespec
and so on. These know only seconds and fractions thereof.

Move inittodr() and resettodr() prototypes to <sys/time.h>.
Retain the names as it is one of the few surviving PDP/VAX references.

Move startrtclock() to <machine/clock.h> on relevant platforms, it
is a MD call between machdep.c/clock.c. Remove references to it
elsewhere.

Remove a lot of unnecessary <sys/clock.h> includes.

Move the machdep.disable_rtc_set sysctl to subr_rtc.c where it belongs.
XXX: should be kern.disable_rtc_set really, it's not MD.

Better naming of fattime conversion functions, they do convert to timespec
after all.

Add 'utc' argument to control if fattimestamps are on UTC or local timezone
calendar.

Add two new functions to convert FAT filesystem format timestamps
to and from struct timespec, to replace the crummy conversion
function which have been copy&pasted into three different
filesystems already.

Apart from general crummyness as indicated by code like:

for (year = 1970;; year++) {
inc = year & 0x03 ? 365 : 366;
if (days < inc)
break;
days -= inc;
}

They also contain specialized crummyness which tries to compensate
for the general crummyness by caching recent conversion results,
with no regard for locking or consistency.

These replacement functions are smaller, O(1) and handle the Y2.1K
leap-year correctly.

Ideally, these functions should live in a module of their own,
which the three offending filesystems would depend on, but the
size is 877 bytes of code (on i386), so that would be false
economy.

Use utc_offset() where applicable, and hide the internals of it
as static variables.

Introduce utc_offset() to capture a calculation currently done all over the
place.

First part of a little cleanup in the calendar/timezone/RTC handling.

Move relevant variables to <sys/clock.h> and fix #includes as necessary.

Use libkern's much more time- & spamce-efficient BCD routines.

The year field is the 4 digit year (eg, 2006), not 'year - 1900' (eg
106). Fix the comment to reflect this.

Add a generic implementation of inittodr() and resettodr(), as well as
a set of helper routines to deal with real-time clocks. The generic
functions access the clock diver using a kobj interface. This is intended
to reduce code reduplication and make it easy to support more than one
clock model on a single architecture.

This code is currently only used on sparc64, but it is planned to convert
the code of the other architectures to it later.