zfsbootmenu-core.sh: do not cache unencrypted keysources

[ahesford]

This needs testing.

I can't think of a single legitimate use for storing ZFS encryption keys on an unencrypted ZFS filesystem. People can do it if they want, but we don't need to help them make bad choices by copying their unencrypted keys via our caching mechanism.

[zdykstra]

Might be nice to add a zerror or zwarn message indicating that the key source can't be used because it's not encrypted.

[ahesford]

Moved from zdebug to zwarn.

[zdykstra]

Tested locally with org.zfsbootmenu:keysource pointing to an un-encrypted dataset; works as expected. No keys are cached and the main menu forces a passphrase input.