nixos/rename: Fix merging multiple destinations.

The renaming of options define the original value for the new attribute
path. This works well if there is only *one* target, but if there are
more, we end up recursing into the attribute set of the option
definition itself.

We now check for that within the parent recursion node (we can't check
that from the subnode, because we lack that information about whether
it's defined multiple times) and if the subnode consist entirely of a
list of definitions, we use mkMerge on it.

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