fattime: fix fattime to timespec conversion of dates beyond 2106-02-06

It turns out that the only conversion issue was in fattime2timespec, where
multiplying the number of seconds in a day by the number of days overflowed
32-bit unsigned int for dates beyond 2106-02-07 06:28:15.

Casting one of the multiplicands as time_t forces a 64-bit multiplication on
systems where time_t is 64-bits and produces no binary changes on the one
remaining system with 32-bit time_t (namely i386).

Since the code is now tested & fixed, this change removes the fixme comments.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44755

fattime: make the test code check beyond 32-bit time_t limits

On systems that have a 64-bit time_t, the test code now exercises the whole
range of fattime. To do this, this commit...

1. replaces the call to random() with two calls to arc4random() to
generate a 33-bit number of seconds in order to cover the entire range of
fattime [1970,2107]. (32-bits stops just short - in January 2106.)
On systems with 32-bit time_t, the extra bits are discarded and only the
time_t expressible range is tested.
2. casts time_t values passed to printf as longs and changes the format
string to match.

Now, the test code builds, runs, and exercises what it can (i.e., the whole
fattime range or the 32-bit time_t subset of it) on both 32-bit and 64-bit
time_t systems.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44754

fattime: make the test code build again

This change...

1. replaces calls to timet2fattime/fattime2timet with calls to
timespec2fattime/fattime2timespec. The functions got renamed shortly
after they landed in the kernel but the test code wasn't updated (see
7ea93e912bf0ef).
2. adds a utc_offset stub.

With this, the test code builds and runs as a 32-bit binary (cc -Wall -O2
-m32 subr_fattime.c).

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44753

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

Remove duplicated empty lines from kern/*.c

No functional changes.

Use const pointers for input data not modified by clock utility functions.

sys/kern: adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

Where appropriate, use the modern terms for the one true time base
(UTC) rather than the archaic (GMT) in comments. Except where the
comments are making fun of people doing this (and pedants who insist
on the new terms).

Fix a comment typo; conversion tables are for leap years, not leap seconds.

Fix typo.

MFC after: 3 days

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

Use ISO C99 integer types in sys/kern where possible.

There are only about 100 occurences of the BSD-specific u_int*_t
datatypes in sys/kern. The ISO C99 integer types are used here more
often.

- Import the HEAD csup code which is the basis for the cvsmode work.

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.