hwclock: make --date=argument less prone to injection

hwclock: make --date=argument less prone to injection

This change should not improve security much.  One hopes hwclock --set is
restricted for root only.  Where hwclock is allowed to run via sudo, or has
setuid setup, there is a pretty easy privilege escalation via subshell.

$ sudo ./hwclock --set --date='2000-10-20$(touch /tmp/hwclock.inject)'

Reviewed-by: J William Piggott <elseifthen@gmx.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>