strptime: Fix day-of-week calculation.

The day-of-week calculation used the raw year value without adjusting
for TM_YEAR_BASE, so it was off by one for 300 years out of every 400;
it just happened to be correct for 1901 through 2000. It also used a
loop where a simple addition would have sufficed.

While here, simplify our version of Gauss's algorithm, and document
that we assume the Gregorian calendar.

MFC after: 1 week
PR: 282916
Reviewed by: imp, allanjude, philip
Differential Revision: https://reviews.freebsd.org/D47977