o/hookstate/ctlcmd: add a new exit code for is-connected for cases when the peer is from the same snap

[jhenstridge]

This PR grew out of discussions about the CUPS snap on the forum:

https://forum.snapcraft.io/t/request-cups-snap-cups-auto-connection-to-of-cups-cups-control-to-cups-admin-and-also-of-the-network-manager-observe-interface/22802/25?u=jamesh

At present, Till's CUPS snap has both a plug and slot for the cups-control interface, with a request that they be auto-connected. At present the plug is not actually necessary to connect to the CUPS socket (that permission comes with the slot), but is needed to verify that utilities like lpadmin to pass the snapctl is-connected check.

As discussed in the forum thread, we could avoid the need for the extra plug if snapctl is-connected grew a new exit code representing "peer is not connected, but belongs to the same snap". So that is what this PR implements.

[stolowski]

Looks good, thank you!

[pedronis]

as discussed, self-connections are not fully unusual, if a snap ships both the server side of a service and client tool, it's possibly a better pattern for the slot permissions to be given only to the services that constitute the server side, in which case the client applications might need a self-connection anyway to talk to the server bits. This code here helps when not doing this, so until there's a clearer use case I would prefer not to go into this direction.