ttylogin service management via /etc/ttys

# `ttylogin` service management via `/etc/ttys`

`ttylogin` services can of course be managed just like ordinary services, enabling them so that they auto-start at bootup and starting and stopping them with normal `system-control` commands. The `preset` command and the normal preset mechanisms can pre-set what services are enabled when they are installed, with (for example) lines in `/etc/system-control/presets/60-ttys.preset` saying:

>     enable ttylogin@ttyv1
>     enable cyclog@ttylogin@ttyv1

However, one might decide to retain the BSD `/etc/ttys` mechanism as the master settings list for all terminals. This is still done with the `preset` command, which can also use the on/off information for individual virtual terminals that is stored in `/etc/ttys` as an alternative source of preset information for enabling/disabling the matching `ttylogin` services.

In order to determine presets using `/etc/ttys` one runs two things:

1. In order to first enable or disable the relevant `ttylogin@ttyv{0,1,2,3,4,5,6,7,8}.service` service one runs

   >     system-control preset --ttys --prefix ttylogin@ ttyv{0,1,2,3,4,5,6,7,8}
   >     system-control preset --ttys --prefix cyclog@ttylogin@ ttyv{0,1,2,3,4,5,6,7,8}

   This takes the preset information given in the matching `ttyv{0,1,2,3,4,5,6,7,8}` records in `/etc/ttys` and applies it to both the main login services and their associated logging services.

2. In order to immediately start or stop the relevant service one also runs

   >     system-control reset ttylogin@ttyv{0,1,2,3,4,5,6,7,8}
   >     system-control reset cyclog@ttylogin@ttyv{0,1,2,3,4,5,6,7,8}

Note: This allows one to list both kernel and user-space virtual terminals in `/etc/ttys`, alongside one another.