sh: heartbeat double 0 fix.

This implements stricter and more compliant knightrider strobing in the
heartbeat handler. While there still seems to be some debate as to
whether the double 0 is "more" correct or not, this updated version
appears to have general consensus. Fixes a long-term "bug".

Signed-off-by: Takashi YOSHII <takashi.yoshii.ze@hitachi.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>