Patch by Tadayoshi Funaba (with some changes) to be smarter about

Patch by Tadayoshi Funaba (with some changes) to be smarter about
guessing what happened when strftime() returns 0.  Is it buffer
overflow or was the result simply 0 bytes long?  (This happens for an
empty format string, or when the format string is a single %Z and the
timezone is unknown.)  if the buffer is at least 256 times as long as
the format, assume the latter.