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: bfb7a31b6a ("rc: Hook zfskeys to the build")
Fixes: 33ff39796f ("Add zfskeys rc.d script for auto-loading encryption keys")

libexec/rc/rc.conf

@@ -67,6 +67,7 @@ rc_conf_files="/etc/rc.conf /etc/rc.conf.local"
 
 # ZFS support
 zfs_enable="NO"		# Set to YES to automatically mount ZFS file systems
+zfskeys_enable="NO"	# Set YES to autoload ZFS encryption keys
 zfs_bootonce_activate="NO" # Set YES to make successful bootonce BE permanent
 
 # ZFSD support