cal: remove arbitrary limit of maxium year being 9999

The change limits year to be unsigned long. In case a year + month name
exceed a column width two lines are used for them.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
tests: check cal works when year is far in future

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
docs: cal: stop telling year 9999 is upper limit

Signed-off-by: Sami Kerola <kerolasa@iki.fi>