Turn "keys" option into an attrset of attrsets.

This now provides options for each key and also converts old style
string-only keys into the new format while emitting a warning.

At the moment only the "text" option is actually supported.

For applying the keys for string-values the permissions attribute is set
to "0600" instead of the default value "0640" in keyOptionsType in order
to correctly replicate the old behaviour even when we implement
permissions and ownership.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>