Horde_Secret: Fix up cookie key name on key change / unset

This was introduced by commit

00191e044206f496ae1f9084deed33d6c7919265

[mms] If secret key is changed, change the associated $_COOKIE key
or else key change may not be recognized in the current page access (Bug #11570).

Signed-off-by: Michael M Slusarz <slusarz@horde.org>
[mms] Correctly clear secret key from cookie data (Bug #13283; thomas.jarosch@intra2net.com).
[mms] Only store keys in cookie if cookies are in use (Bug #13284; thomas.jarosch@intra2net.com).
Remove remaining elements of freecode support
Released Horde_Secret-2.0.3
Development mode for Horde_Secret-2.0.4