Bug#825394: also mosh

[Shish]

`mosh-server` should be added to the list of processes which should be
exempt from this behaviour, as it is based on the principle of "ssh
into remote host, start daemon, exit ssh, continue speaking to
daemon".

As a workaround, I'm running it in a new session explicitly:

  mosh --server "systemd-run --scope --user mosh-server" $hostname

To make this more googlable, the error message which happens when
systemd kills mosh-server is:

  mosh: Nothing received from server on UDP port 60001. [To quit: Ctrl-^ .]