log: introduce "none" logging backend

In initrd, the NM service should write logs to both the active console and the journal. To achieve that, NetworkManager is started with --debug to output logs to stderr, and the systemd unit is configured with StandardError=tty to redirect messages to the console. NM is configured with logging.backend=journal to also save logs in the journal. The problem of this approach is that when the system doesn't have a console, the NM service fails to start [1].

This can be solved by using StandardError=journal+console which forwards messages from stderr to journald (which in turn also writes them to the console if available). To avoid duplicates entries in the journal, we need a way to disable all logging backends in NM and only write to stderr.

Introduce a none logging backend. Of course, it's not recommended to start NM without logging; therefore the none backend can't be used as build-time default. none only makes some sense when also starting NM with --debug.

[1] https://github.com/coreos/fedora-coreos-tracker/issues/943