It's easy to get stuck in an infinite loop in the hush shell:

while true; do sleep 1; done

The 'sleep' command will check for ctrl-c with the ctrlc() function. This
will abort the sleep command. Hush then checks for ctrl-c again in the
loop. The ctrl-c in the buffer has already been eaten by the sleep
command, so the loop will continue.

With this patch we remember the presence of a ctrl-c character in a
variable instead of checking for a new character each time. The
variable must be resetted explicitly by calling ctrlc_handled() which
will be called by the shell in the outer loop.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>