"debug" was documentation in `man NetworkManager.conf` as a valid
logging backend. However, it was completely ignored by
nm_logging_syslog_openlog().
In fact, it makes not sense. Passing debug = TRUE to
nm_logging_syslog_openlog(), means that all messages will be
printed to stderr in addition to syslog/journal. However, when
NetworkManager is daemonizing, stderr is closed.
Whether NetworkManager is daemonizing depends entirely on command
line options --no-daemon and --debug. Hence, the logging backend "debug"
from the configuration file either conflicts or is redundant.

Also, adjust logging backend description in `man NetworkManager.conf`.

Also, log a warning about invalid/unsupported logging backend.

(cherry picked from commit 2ccf6168)