logging: warn about invalid logging backends and drop "debug" backend
"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.
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