keyhandler: Do not serialise keyhandlers; increase scratch array size.

keyhandler: Do not serialise keyhandlers; increase scratch array size.

Although serialising keyhandlers is safer, and in particular
protects access to shared heyhandler_scratch[], in debug scenarios it
is probably better to 'have a go' when requested - and assume the user
knows what they are doing.

Meanwhile, increase scratch array size to 1024. That's enough for more
than a dozen lines of 80-column text, and should be plenty in any
practical situation.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>