Fix j2day() to behave sanely for negative Julian dates.

Fix j2day() to behave sanely for negative Julian dates.

Somebody had apparently once figured that casting to unsigned int would
produce the right output for negative inputs, but that would only be
true if 2^32 were a multiple of 7, which of course it ain't.  We need
to use a signed division and then correct the sign of the remainder.

AFAICT, the only case where this would arise currently is when doing
ISO-week calculations for dates in 4714BC, where we'd compute a
negative Julian date representing 4714-01-04BC and then do some
arithmetic with it.  Since we don't even really document support for
such dates, this is not of much consequence.  But we may as well
get it right.

Per report from Vitaly Burovoy.