merge-nodes: Allow merging from supervised source nodes

[xearl4]

Description

Currently, when running merge-nodes with a "from"/source node which is supervised, it'll abort with the following error message:

source appears to be a supervised node - only merging of remote smeshers is supported

It would be nice if supervised nodes are allowed as "from"/source nodes having their local.key copied under as <hex smesher ID>.key into the "to"/target node.

So, for example, given a supervised (1:1) node which manages the smesher ID 7b0bd169d332f894d45ab9fc5d50595f95a799a8136cde733f46c0eec50c3d24. merge-nodes would copy this "from" node's local.key to the "to" node as 7b0bd169d332f894d45ab9fc5d50595f95a799a8136cde733f46c0eec50c3d24.key.