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
60 jobs for bg/log-none in 2 minutes and 16 seconds (queued for 12 seconds)