defines strftime(), but can also define wcsftime(); add optional
test package; speed up %Y handling.
* libc/time/wcsftime.c: New file, defining wcsftime() (albeit
indirectly by including strftime.c) and its documentation.
* libc/time/time.tex: Enhance tm_isdst explanation, change strftime
description to match modified description in strftime.c
* libc/time/Makefile.am: Add wcsftime.c and wcsftime.def. Add a rule
so that wcsftime.o gets rebuilt when strftime changes.
* libc/time/Makefile.in: Regenerate.
* libc/include/wchar.h (wcsftime): Declare.
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