ncal(1): handle Sweden's transitional calendar (1700-1712)HEADmain

Sweden used a unique intermediate calendar during 1700-1712.  In 1700,
Sweden omitted the Julian leap day (Feb 29, 1700) to begin a gradual
transition to the Gregorian calendar.  The transition was abandoned
mid-way due to the Great Northern War; to revert to the Julian calendar,
an extra day "Feb 30, 1712" was added.  Sweden finally adopted the
Gregorian calendar in 1753 (already handled correctly by ncal).

ncal previously displayed Feb 1700 as a 29-day month and Feb 1712 as a
29-day month, both incorrect for the Swedish (-s SE) calendar mode.

Add flag_sweden, which is set when the SE country code is selected (via
-s SE or the locale) and cleared by -J, since -J selects the plain
Julian calendar.  When active, sndaysr() and sdater() apply the one-day
offset that characterised the Swedish transitional calendar:

  - Feb 1700: displayed with 28 days (no leap day)
  - 1700-03-01 to 1712-02-30: dates run one day ahead of Julian, so
    weekdays are correctly anchored to the actual solar day
  - Feb 1712: displayed with 30 days (the unique "Feb 30" day)
  - 1712-03-01 onward: back to Julian until the 1753 Gregorian switch

Add ATF tests covering the 3-month and Julian-day views around both
boundaries and the -J override.

Reported-by:	John Costello <https://joxn.org>
Signed-off-by:	Juan Michelini <juan@juan.com.uy>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Reviewed by:	asomers, js
MFC after:	2 weeks
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2353

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