console: add user console daemon

console: add user console daemon

This adds a first draft of systemd-consoled. This is still missing a lot
of features and does some rather primitive rendering. However, it shows
the direction this code is going and serves as basis for further testing.

The systemd-consoled binary should be run as `systemd --user' unit. It
automatically picks up any session marked as Desktop=SYSTEMD-CONSOLE.
Therefore, you can use any login-manager you want (ranging from /bin/login
to gdm) to create sessions for systemd-consoled. However, the sessions
managers must be prepared to set the Desktop= variable properly.

The user-session is called `systemd-console', only the daemon providing
the terminal environment is called `systemd-consoled' (mind the 'd').

So far, only a single terminal session is provided on each opened
user-session. However, we support multiple user-sessions (even across
multiple seats) just fine. In the future, the workspace logic will get
extended so you can have multiple terminal sessions in a single
user-session for easier access.

Note that this is still experimental! Instructions on how to run it will
follow shortly.