1) Microoptimize %p case.

1) Microoptimize %p case.
2) Implememt %u for GNU compatibility.
3) Don't forget to advance buf for %w/%u.
4) Fail with incomplete week (week 0) request and no such week in the
year.
5) Fix yday formula when Sunday requested and the week started from Monday.
6) Fail with impossible yday for incomplete week (week 0) and direct %w/%u
request.
7) Shift yday/wday to the first day of the year, if incomplete week
(week 0) requested and no %w/%u used.

MFC after:      7 days