cal: correct gregorian week numbering

cal: correct gregorian week numbering

Jan 1 is always First week, and year always has 53 weeks.  The week 53
may be cut short, e.g., it may and often has fewer than 7 days.  Every
year 28 year intervals US week numbering continues all the way to 54th
week, such as 1972, 2000, and 2028.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1249486
Reported-by: Michal Toth
Signed-off-by: Sami Kerola <kerolasa@iki.fi>