bug#21904: date->string duff ISO 8601 format for non-4-digit years

[Zefram]

The date->string function from (srfi srfi-19), used on ISO 8601 formats
"~1", "~4", and "~5", gets the formatting of year numbers wrong when the
year number doesn't have exactly four digits.  There are multiple cases:

scheme@(guile-user)> (date->string (julian-day->date 1500000 0) "~1")
$1 = "-607-10-04"
scheme@(guile-user)> (date->string (julian-day->date 1700000 0) "~1")
$2 = "-59-05-05"
scheme@(guile-user)> (date->string (julian-day->date 1720000 0) "~1")
$3 = "-4-02-05"

For year numbers -999 to -1 inclusive, date->string is using the minimum
number of digits to express the number, but ISO 8601 requires the use
of at least four digits, with zero padding on the left.  So one should
write "-0059" rather than "-59", for example.  Note that this range is
also affected by the off-by-one error in the selection of the year number
that I described in bug #21903, but that's not the subject of the present
bug report.  Here I'm concerned with how the number is represented in
characters, not with how the year is represented numerically.

scheme@(guile-user)> (date->string (julian-day->date 1722000 0) "~1")
$4 = "2-07-29"
scheme@(guile-user)> (date->string (julian-day->date 1730000 0) "~1")
$5 = "24-06-23"
scheme@(guile-user)> (date->string (julian-day->date 2000000 0) "~1")
$6 = "763-09-18"

For year numbers 1 to 999 inclusive, again date->string is using the
minimum number of digits to express the number, but ISO 8601 requires the
use of at least four digits.  If no leading "+" sign is used then the
number must be exactly four digits, and that is the appropriate format
to use in this situation.  So one should write "0024" rather than "24",
for example.

The year number 0, representing the year 1 BC, logically also falls into
this group, and should be represented textually as "0000".  Currently this
case doesn't arise in the function's output, because the off-by-one bug
has it erroneously emit "-1" for that year.

scheme@(guile-user)> (date->string (julian-day->date 10000000 0) "~1")
$7 = "22666-12-20"
scheme@(guile-user)> (date->string (julian-day->date 100000000 0) "~1")
$8 = "269078-08-07"

For year numbers 10000 and above, it is necessary to use more than four
digits for the year, and that's permitted, but ISO 8601 requires that
more than four digits are preceded by a sign.  For positive year numbers
the sign must be "+".  So one should write "+22666" rather than "22666",
for example.

The formatting of year numbers for ISO 8601 purposes is currently only
correct for numbers -1000 and lower (though the choice of number is off
by one) and for year numbers 1000 to 9999 inclusive.

-zefram