Emacs 27 added Type=notify support and updated the service definition to
remove the use of `emacsclient' to kill the service. Emacs 28 changes
the `StartupWMClass' in emacsclient.desktop to `Emacsd'. Update our
emacs.service and emacsclient.desktop definitions to match upstream
changes.
When killing emacs.service, the socket is removed, and subsequently
starting the service manually results in a service without a socket.
Prevent this by adding `RefuseManualStart=true' to the service's Unit
definition.
Drop Emacs 26 support as it is no longer shipped in nixpkgs. Update the
tests to verify the following configuration scenarios:
- Emacs version: 27, 28
- Socket activation: disabled, enabled
