etc/defaults/rc.conf: set default of zfskeys_enable to NO

This addresses the following boot message:

/etc/rc: WARNING: $zfskeys_enable is not set properly - see rc.conf(5).

Reported by: Mark Millard
Sponsored by: Modirum MDPay
Sponsored by: Klara Inc.
Fixes: bfb7a31b6a22 ("rc: Hook zfskeys to the build")
Fixes: 33ff39796ffe ("Add zfskeys rc.d script for auto-loading encryption keys")