banned.h: mark ctime_r() and asctime_r() as banned

banned.h: mark ctime_r() and asctime_r() as banned

The ctime_r() and asctime_r() functions are reentrant, but have
no check that the buffer we pass in is long enough (the manpage says it
"should have room for at least 26 bytes"). Since this is such an
easy-to-get-wrong interface, and since we have the much safer strftime()
as well as its more convenient strbuf_addftime() wrapper, let's ban both
of those.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>